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Preliminary geomorphological and hydrographical characterization of a circular structure in the Marche Region (Central Italy) and its possible origin

Caractérisation géomorphologique et hydrographique préliminaire d'une structure circulaire dans la région des Marches (Italie centrale) et son origine possible
Sabrina Colucci et Cristiano Fidani

Résumés

Les images Google Earth d'une zone incluse dans une partie sud de la région des Marches, au centre de l'Italie, ont révélé une morphologie circulaire consistant en un système annulaire de collines et de dépressions concentriques par rapport à une élévation centrale. Cette morphologie est aussi caractérisée par des couleurs différentes de la végétation dans les terres cultivées et par son réseau hydrographique. Les données cartographiques et géologiques de la région des Marches ont été examinées et une étude morphométrique a montré que le bord externe de cette structure est elliptique, avec le grand axe orienté SO-NE. Plus précisément, cette ellipse a une hauteur moyenne de 373 m, un diamètre moyen de 3,75 km et une surface interne de 10,5 km2. Quatre origines ou processus qui auraient pu générer cette forme annulaire sont étudiés: un astroblème, l’organisation hydrographique du bassin, un brachyanticlinal et la formation d'un dôme diapique. Une analyse morphologique a été réalisée sur la base des indices morphométriques. Les paramètres hydrographiques ne satisfont pas à la loi de Hack, excluant l'origine hydrologique. De même, les relevés géophysiques disponibles ne concordent pas avec les hypothèses d'un impact et d'une brachyanticlinal. La présence d'extrusions de boue dans la zone étudiée suggère une origine liée à la formation d’un diapir argileux.

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Notes de la rédaction

Manuscript received on November 11, 2020, revised manuscript received on January 19, 2022, paper accepted on February 16, 2022

Texte intégral

The authors would like to thank the municipal administrations of Sant'Angelo in Pontano, Falerone, Monte Vidon Corrado, Loro Piceno and Penna San Giovanni for providing the study with geotechnical material and information on mud volcanoes. The authors are grateful to two anonymous referees who have greatly improved the manuscript. The authors would also like to thank Angela Faraone for her support.

1. Introduction

1The increasing quality of very high-resolution space imagery has strongly contributed to the identification of circular morphologies like impact structures (Garvin et al., 1995) or volcanic craters (Cengiz et al., 2006). The photos analyzed in this work are satellite optical images, showing a circularly shaped morphology located about 35 km west of the Adriatic Sea coast and 15 km east of the Apennine Chain in Marche Region, Italy, with coordinates 43°06' N and 13°25' E (fig. 1). It is made up of two concentric circles, evidenced by differences between the green colors of richer vegetation in the moats, and the greens and browns on cultivated uplands. The general morphology of the southern basin area in the Marche Region where the structure is located, is characterized by rolling hills. The degradation of rounded hills is separated by valleys that descend between the Apennines to the west and the Adriatic Sea to its east (Micarelli et al., 2014), so that the circular landform appears as an anomaly in this environment. The area of this morphological feature, including the external ring, is estimated to be about 50 km2, inside the dotted circle in Figure 1. To date, neither impact crater morphologies nor similar landforms have been reported in Italy (French, 1998).

Fig. 1 - Google Earth image of Marche Region, Central Italy (A), delimited by a dashed line, where the structure was observed (B), 10X magnification of the structure; dashed circle (C): outer limit of the whole annular landform.
Fig. 1 - Image Google Earth de la région des Marches, au centre de l'Italie (A) où la structure a été observée (B), agrandissement 10X de la structure, délimitée par un cercle en pointillé (C).

Fig. 1 - Google Earth image of Marche Region, Central Italy (A), delimited by a dashed line, where the structure was observed (B), 10X magnification of the structure; dashed circle (C): outer limit of the whole annular landform.Fig. 1 - Image Google Earth de la région des Marches, au centre de l'Italie (A) où la structure a été observée (B), agrandissement 10X de la structure, délimitée par un cercle en pointillé (C).

2The circularity and dimensions of the structure initially suggested the astroblem hypothesis as the main one to be subjected to an accurate analysis based on the available data. Impact cratering was recognized as a significant geological process which contributed to the evolution of all solid planetary bodies (Melosh and Ivanov, 1999). The Earth's impact craters were initially mainly recognized by their association with meteoric material. Several larger structures, which were later known to be of impact origin, were dismissed as being “crypto-volcanic” (Melosh, 1989) during the first half of 20th century. Since 1955, the number of accepted terrestrial craters has risen at a rate of about 2-4 per year and recently has reached about 190 cases; as listed in the Earth Impact Database (2018). Recent discoveries have been the result of photographic satellite analyses, remote sensing of gravity and geomagnetic fields, as well as recently carried out geophysical surveys (Garvin et al., 1995; Koeberl et al., 2005; Ghoneim, 2009; Xiao et al., 2018). A methodology for detecting automatically circular structures from space had already been proposed (Taud and Parrot, 1992). However, remote sensing tools cannot provide diagnostic evidence of meteoric impact or volcanic formations (Grieve, 1991). Rock sample and geological analyses are also necessary to support their extra-terrestrial or endogenous origins (Grieve, 1991; Stoppa, 2006; Cigolini et al., 2012).

3Analyzing shape indexes in comparison with those of terrestrial craters and considering geophysical data, inconsistencies emerged. Indeed, rings of hills, lakes, or isolated circular areas of intense rock deformation appeared in otherwise undeformed bedrock (French, 1998) were interpreted not only as results of impact cratering, but also as magmatism, diapirism, mud volcanism, karst dissolution, or glacial erosion (Schmieder et al., 2009). To date, space programmes have focused more upon volcanology due to the physical similarities between impact and volcanic craters (Piero, 1976), and to the existence of volcanic structures on other planets (Garvin et al., 2000). Salt diapirism has been suggested in geophysical studies of similar dimension domes (Jackson et al., 1998), as well as the presence of over-pressured sediments leading to mud extrusions (Kopf, 2002). Automatic analysis of karst depressions has improved the remote identification and delineation of dolines using a high-resolution Digital Elevation Model (DEM) (Pardo-Iguzquiza et al., 2016). Moreover, erosion processes due to rain and wind significantly have modified the above cited structures on Earth, and must always be considered when performing a morphometric analysis (Sharp, 1980). Thus, after describing topography and geology of the area in Section 2, and the methodology in Section 3, results concerning geophysics and morphometric analysis were calculated in Section 4. Discussion in Section 5 extensively evaluated results in relation to the first hypothesis of the astrobleme, which raised many doubts. So that, the hydrographic, the brachyanticline, and the diapiric hypotheses were also taken into consideration, where only the last hypothesis agree with the available data. The aims of our investigation are to describe the observed morphology and to provide constraints regarding its nature and origin.

2. Topographic description and geological settings

4Two concentric circles are evidenced by moats located about 0.9 km and 2.7 km from the center of the morphology. The highest hill altitudes in the western part exceed 500 m in the Sant'Angelo in Pontano Village, whereas the lowest hill altitudes reach only 300 m towards the NE part of the morphology in the Contrada San Martino (fig. 2). The rim diameter of these hills is on average D = 3.75 km. The circular shaped morphology was dug out by the moats called Tifo to the north, Faverchiara to the east, and two branches forming the creek called Ete Morto. The Ete Morto cuts through the structure marking the lowest altitude at 159 m. The landform is bounded by the Fiastra River to the west and the Salino creek to the south. The hills beyond the Tifo Valley, observable in the north and NE parts of the morphology, see Figure 2, form a second annular ring that has heights between 300 and 400 m. The rim of these external hills is on average 3.7 km from the center of the morphology. The studied landform is located in the Marche Region of Italy, close to the transverse fault Fiastrone-Fiastrella (fig. 3) of the Laga Basin (Ricci Lucchi, 1975). The inner part of the multi-ring structure was found in the administrative territory of Sant'Angelo in Pontano and Falerone municipalities, on the border between Fermo and Macerata Provinces. Centered at 43°06'16.1'' N and 13°25'30.6'' E with a focus elevation of 352 m asl, it is located principally in the municipality of Sant'Angelo in Pontano, while the external ring occupies a minor area of the municipalities of: Penna San Giovanni, San Ginesio, Loro Piceno, Montappone, and Gualdo. The stratigraphy of this area is found in the Italian Geological Survey 1:50,000 scale geological map at Foglio 314 Montegiorgio (Micarelli et al., 2014), reported in Figures 3 and 4, which is based on modern stratigraphical concepts (CARG collaboration, 2017). We also used the Topographic Regional Maps F°124 Quadrante II SE and F°125 Quadrante III SW, scale 1:25,000.

Fig. 2 - Google Earth image with elevation 3X showing two annular rings viewed in 245°N direction from a height of 2.7 km; colored altitude map in the upper left; Contrada San Martino is the elongated central hill that forms the north ridge of the inner ring; the Apennine chain is at the top background.
Fig. 2 - Image Google Earth avec élévation 3X, montrant deux anneaux vus vers 245 ° N d'une hauteur de 2,7 km ; carte hypsométrique colorée en haut à gauche ; Contrada San Martino est la colline centrale allongée qui forme la crête nord de l'anneau intérieur ; la chaîne des Apennins est en arrière-plan au-dessus.

Fig. 2 - Google Earth image with elevation 3X showing two annular rings viewed in 245°N direction from a height of 2.7 km; colored altitude map in the upper left; Contrada San Martino is the elongated central hill that forms the north ridge of the inner ring; the Apennine chain is at the top background.Fig. 2 - Image Google Earth avec élévation 3X, montrant deux anneaux vus vers 245 ° N d'une hauteur de 2,7 km ; carte hypsométrique colorée en haut à gauche ; Contrada San Martino est la colline centrale allongée qui forme la crête nord de l'anneau intérieur ; la chaîne des Apennins est en arrière-plan au-dessus.

5The characteristic lithologies of this area are those of the Laga Basin, the largest of the “minor basins” in Umbria-Marche (Centamore et al., 1982). The Laga Basin is the largest and youngest foreland sedimentary basin of the Central Apennines fold and thrust belt (Artoni, 2003). During the Pliocene-Pleistocene Era, tectonic activity influenced the basin morphology dividing it into five main areas characterized by different evolution. These, from the north to the south are Ancona, Macerata, Fermo, Teramo, and Chieti areas (Boccaletti et al., 1986). The Fermo area, containing the morphology under study, is the most depressed area of the basin; it is characterized by a succession of clay layers deposited in bathyal environment occurring throughout the Messinian Period (Micarelli et al., 2014). Moreover, evaporites were deposited in the northernmost area, while an euxinic environment reigned in the more depressed southern area. The Messinian turbiditic deposits belong to the Laga Formation, characterized by mainly pelitic and arenaceous lithofacies containing gypsum intercalations. The turbidites of the evaporite member are considered as channeled deposits, constituting the filling of channels and depressions of structural origin, which are typically narrow and elongated (Mutti and Ricci Lucchi, 1981). Whereas non-channeled deposits are considered results of momentary closures for sedimentation both in a channel or in a depression. The Laga Formation is overlain by the Colombacci Formation, which is composed of sandy-to-silty layers intercalated with calcareous levels. These deposits belong to the Quaternary Blue Clays Formation and consist of alternated arenaceous-pelitic and pelitic-arenaceous lithofacies (Mutti and Ricci Lucchi, 1981).

Fig. 3 - Structural map of the area covered by the annular landform; main geological formations of the Laga Basin area: from Ricci Lucchi et al. (1975) and Micarelli et al. (2014); a circular dashed line surrounds the entire morphology.
Fig. 3 - Carte structurale de la zone couverte par la structure annulaire. Principales formations géologiques de la région du bassin de Laga : d’après Ricci Lucchi et al. (1975) et Micarelli et al. (2014) ; une ligne en pointillé entoure toute la structure.

Fig. 3 - Structural map of the area covered by the annular landform; main geological formations of the Laga Basin area: from Ricci Lucchi et al. (1975) and Micarelli et al. (2014); a circular dashed line surrounds the entire morphology.Fig. 3 - Carte structurale de la zone couverte par la structure annulaire. Principales formations géologiques de la région du bassin de Laga : d’après Ricci Lucchi et al. (1975) et Micarelli et al. (2014) ; une ligne en pointillé entoure toute la structure.

1. Laga Basin. Blue Clays; 2. Monte dell'Ascenzione Member; 3. Spugnone Member; 4. Pelitic; 5. Colombacci Formation. Laga Formation: 6. Postevaporitic Member; 7. Evaporitic Member. 8. Camerino Basin; 9. Internal Marche Basin; 10. External Marche Basin; 11. Fault; 12. Anticlinal; 13. Synclinal; 14. Thrust.
1. Bassin de Laga. Argiles Bleues ; 2. Membre du Monte dell'Ascensione ; 3. Membre du Spugnone ; 4. Pélitique ; 5. Formation à Colombacci. Formation de la Laga : 6. Membre Postvaporitique ; 7. Membre Évaporitique ; 8. Bassin de Camerino ; 9. Bassin Interne des Marches ; 10. Bassin Extérieur des Marches ; 11.
Défauts ; 12. Anticlinal ; 13. Synclinal ; 14. Poussée.

6From a tectonic point of view, the structures of the Marche-Umbria are defined as chain folds and thrust faults generated by compressional stress during the Miocene-Early Pliocene period (Tavarnelli, 1997). The compression direction is mainly SW-NE, with movement of the rock mass to the NE. Since the Late Pliocene, extensional tectonics, with a SW-NE direction, has had a chain effect starting from the late Pliocene, generating normal faults trending NW-SE. These faults, which are visible especially in the southern part of the Marche and Umbria Regions, are evidenced by blocks gradually lowering to SW. The geometry (Bally et al., 1986), seismicity and rheology of the crust and mantle lithosphere along the trace of the CROP 03 and DSS 78 profiles allow an identification at a lithospheric scale (Lavecchia et al., 2003; Lavecchia et al., 2007) of a seismogenic structure named Adriatic Thrust SW Dipping. The seismicity is relatively deep in this area, down to the depth of 20–25 km, as shown in Figure 8 in the publication by Lavecchia et al., (2007). Figures 3 and 4 show an anticline fold near the Sant'Angelo in Pontano Village with an NW-SE axis which is limited by two high angle thrust faults. This fold determines the movement of the evaporitic member on the young post-evaporitic one and post-evaporitic Laga member on the Blue Clays. The anticline in the Colombacci Formation is elongated in the NE direction, see Figure 5A. To the north of the morphology, there are more hypothetical faults with SW-NE directions (fig. 3).

Fig. 4 - Stratigraphic details of section A-A (fig. 3) across the annular structure; adapted from the Marche Geological Survey, scale 1:50000, at www.isprambiente.it. LAG – Laga Formation, Miocene sup., Messinian; FCO – Colombacci formation, Miocene sup., Messinian; FAA – Clays formation, Pliocene inf., Pleistocene sup.
Fig. 4 - Détails stratigraphiques de la section A-A (Fig. 3) à travers la structure annulaire ; adaptée de Marche Geological Survey, échelle 1:50000, sur www.isprambiente.gov.it. LAG – Laga Formation, Miocene sup., Messinian ; FCO – Colombacci formation, Miocene sup., Messinian ; FAA – Clays formation, Pliocene inf., Pleistocene sup.

Fig. 4 - Stratigraphic details of section A-A (fig. 3) across the annular structure; adapted from the Marche Geological Survey, scale 1:50000, at www.isprambiente.it. LAG – Laga Formation, Miocene sup., Messinian; FCO – Colombacci formation, Miocene sup., Messinian; FAA – Clays formation, Pliocene inf., Pleistocene sup.Fig. 4 - Détails stratigraphiques de la section A-A (Fig. 3) à travers la structure annulaire ; adaptée de Marche Geological Survey, échelle 1:50000, sur www.isprambiente.gov.it. LAG – Laga Formation, Miocene sup., Messinian ; FCO – Colombacci formation, Miocene sup., Messinian ; FAA – Clays formation, Pliocene inf., Pleistocene sup.

7The study area has extremely varied geomorphology, in relation with the lithostructural characteristics of the land and the Quaternary evolutionary history (Micarelli et al., 2014). In the westernmost part, where the Laga Basin emerged, the greater relief amplitudes are observed and the mass wasting seems to have been intensified by gravity. It is evidenced by the occurrence of numerous landslides (fig. 5B), mainly characterized by debris flows and collapses subordinated to phenomena of translation and rotational translation (Gentili and Panbianchi, 1989). The circular morphology includes the municipal territories of Sant'Angelo in Pontano, Gualdo, Ripe di San Ginesio, Loro Piceno, Montappone, Falerone, and Penna San Giovanni. It is delimited by the circular dashed lines in Figure 3 which corresponds to the area delimited by the circular dashed line in Figure 1. This area is nearly entirely covered by crops and rural farmlands, which are divided by ditches at a scale of hundred of meters that delineate the hydrographic network where vegetation is richer.

Fig. 5 - Two photos taken from the centre of the morphology, one to the east (A) and the other to the west (B); the points of view are indicated by the respective colored altitude maps.
Fig. 5 - Deux photos prises depuis le centre de la morphologie, l'une à l'est (A) et l'autre à l'ouest (B) ; les points de vue sont indiqués par les cartes d'élévation colorée respectives.

Fig. 5 - Two photos taken from the centre of the morphology, one to the east (A) and the other to the west (B); the points of view are indicated by the respective colored altitude maps.Fig. 5 - Deux photos prises depuis le centre de la morphologie, l'une à l'est (A) et l'autre à l'ouest (B) ; les points de vue sont indiqués par les cartes d'élévation colorée respectives.

3. Methodology

8A large-scale morphological feature may be revealed in many ways such as from field geology, remote sensing, geophysics, and from geomorphological analysis. Moreover, verification of its origin can be obtained only by identification of specific process-produced features in the rock formations themselves (French and Koeberl, 2010). For example, shock-metamorphic effects and other geochemical signatures are measurable from field work in impact structures. However, rock microscopic morphology and geochemical surveys were not considered in the present study. We rather used existing geological surveys of the region, combined with morphometric analysis, hydrography, and geophysical data such as gravitational and geomagnetic anomalies.

9In order to make a morphometric analysis, the area was sampled using cartography and digital image analyses from satellite data. The circular structure was first outlined both manually from the topographic map with a scale of 1:10,000, relative to the sections of: Ripe di San Ginesio n. 314050, Falerone n. 314060, Servigliano n. 314100 and Gualdo n. 314090, and from Google Earth imagery to produce polygons encompassing the circular shape along the rim. The area, perimeter, and lengths of the major and the minor axes of the circular feature were measured also utilizing polygons. Due to various uncertainties in the estimation of such parameters by Google Earth digital images, the triangular irregular network format TINITALY/01 was used for the Italian DEM in the UTM 32 WGS 84 coordinate system (Tarquini et al., 2007). This is the most accurate DEM covering Italy, created by using heterogeneous elevation data sets obtained from existing digital cartography (Tarquini et al., 2012). The table w47585_s10 was obtained from the download area (tinitaly.pi.ingv.it/download.html), which comprises the southern part of the Marche Region. Data viewing, editing and analysis were performed by QGIS free platform (at www.qgis.org/).

10Google Earth images helped to enhance the various image angles to identify the physical characteristics of the morphology, see for example Figure 2. The colors in Figure 1 differentiate plant growth and soil types. The breached rims appear as moats crossing the lower part of the ridges, as shown in Figure 6. Finally, the possible detection of geophysical signatures such as magnetic (Cassidy and Renard, 1996) and gravity (Pilkington and Grieve, 1992) anomalies are useful to reveal buried meteorite fragments. The magnetic anomaly map of Italy (Tontini et al., 2004) together with the gravity anomaly map of Italy (Tiberti et al., 2005) were reported (fig. 7A-B). Moreover, data from many local studies that included municipal geological reports as well as samples from civil planning, were utilized to gain a more complete description of the area (Stortini 1997; Cutini, 2012; Micarelli et al., 2014). Technical reports relative to municipalities of Sant'Angelo in Pontano (Stortini, 1995; Stortini 1998) and Falerone (Procaccini, 2010), up to a scale of 1:2,000, were used to add descriptions of several locations around the municipalities.

Fig. 6 - Hydrography of the study area at a scale of 1:100,000 by Geoportale Nazionale. Ministero dell'Ambiente at http://www.pcn.minambiente.it/​mattm/​ (2011).
Fig.6 - Hydrographie de la zone d’étude à l'échelle 1:100000 par Geoportale Nazionale. Ministero dell'Ambiente à http://www.pcn.minambiente.it/​mattm/​ (2011).

Fig. 6 - Hydrography of the study area at a scale of 1:100,000 by Geoportale Nazionale. Ministero dell'Ambiente at http://www.pcn.minambiente.it/​mattm/​ (2011).Fig.6 - Hydrographie de la zone d’étude à l'échelle 1:100000 par Geoportale Nazionale. Ministero dell'Ambiente à http://www.pcn.minambiente.it/​mattm/​ (2011).

11The morphometric analysis was performed by applying both the sets of shape indexes and hydrographic basin indexes to the retrieved parameters. The shape indexes are dimensionless indices that provide a rough idea of the planimetric shape of the studied morphology and were defined as: aspect ratio, elongation, and isoperimetric circularity. Aspect ratio (AR) is defined as the ratio of a crater’s diameters:

12where Dminor is the length of the crater’s minor axis and Dmajor is the length of the crater’s major axis. Here the minor axis is measured as the axis perpendicular to the major axis running through the center point. An aspect ratio of 1 represents an equal shape around the center point; as the disparity between the two axes increases, the aspect ratio decreases away from 1. A description of asymmetrical shapes can be obtained from elongation (EL) which is defined as:

13where S is the area inside the crater rim calculated by the digitized polygon. Elongation is the ratio between the area of a circle, whose diameter is equal to the major axis, and the feature area. A circle has EL equal to 1 and more elongated shapes have smaller values. Curvature variations of a given morphology were quantified using isoperimetric circularity (IC), that is defined as the ratio between the area of a crater polygon and the area of a circle with the same perimeter.

14where P is the rim morphology perimeter. A circular rim possessing a constant curvature has IC = 1 while shapes where the curvatures vary have IC < 1. IC is a measure of the morphology compactness. Depth to diameter ratio as follow:

15It is calculated using the available value d for depth along the major axis and where D is the average diameter. Depth was recorded with respect to the highest point of the morphology and is useful in making a comparison between dd ratios of Earth craters and the studied morphology. Given that the structure is also the hydrographic basin of the Ete Morto Valley, several indices were defined based upon hydrography and were considered together with the link between the mainstream length (Lb) and the basin area (Sb). The link is known as the Hack Law (Hack, 1957):

16The Gravelius index (Ig) (Gravelius, 1914), which is the relationship between the perimeter Pb of the basin and the perimeter of a circle with area Sb equal to the basin area, indicates the degree of irregularity of the contour of the morphology:

17The closer this ratio is to 1, the greater its areal approximation to a circular shape. The elongation ratio (Ra) is the ratio between the diameter of the circle, having the same surface area Sb as that of the basin, and the mainstream length (Schumm, 1956):

Fig. 7 - Sea level total magnetic field anomaly map
Fig. 7 - Carte des anomalies du champ magnétique total au niveau de la mer

Fig. 7 - Sea level total magnetic field anomaly map Fig. 7 - Carte des anomalies du champ magnétique total au niveau de la mer

(A), from Chiappini et al. (2000), and gravity anomaly map with three provinces (B), downloaded at www.isprambiente.gov/Media/milione_grav/milionegrav_2004/milione.html, both for the region of Central Italy where the ring structure is inside the dashed circle.
(A), tirée de Chiappini et al. (2000), et carte des anomalies gravimétriques avec trois provinces (B), téléchargées sur www.isprambiente.gov/Media/milione_grav/milionegrav_2004/milione.html, toutes deux pour la région d'Italie centrale où la structure annulaire est à l'intérieur du cercle en pointillé.

4. Results

4.1. Stratigraphy and geophysics

18The studied landform is located in the sub-Apennine belt which is mainly hilly with an abundance of cultivated land, separated by ditches a few meters wide, and typical vegetation such as oak woods, hygrophilous vegetation along the water courses, and broom shrubs that colonize the abandoned fields. The Sant'Angelo in Pontano municipality is part of the Fermo territory, south-east of the line of Fiastrone-Fiastrella, where evaporites are replaced by silico-clastic turbidites and chalk (fig. 4). Specifically, clays date back to Lower Pliocene Era (Cantalamessa et al., 1986), about 5 Myears, and are made up of arenaceous strata that underwent diagenetic process and interleaved by: arenaceous-pelitic levels in the middle thin layers, levels of black bituminous marls of euxinic environment, and in the basal part a horizon consisting of turbiditic gypsarenites with a high siliciclastic content (Parea & Ricci Lucchi, 1972). Detailed geological surveys at 1:10,000 scale of the study area were carried out based on several sections of the Regional Technical Map (n. 314050 Ripe di San Ginesio, n. 314060 Falerone, n. 314090 Gualdo and n. 314100 Servigliano). These evidence gray-blue clays with thin stratifications, where arenaceous levels of variable thickness and frequency are observed on the reliefs that border the alluvial plain of the Fiastra River and on the reliefs to the east and south-east of the Sant'Angelo in Pontano village. Sandstone and sand, more or less cemented in layers of various thickness up to a few meters, sometimes amalgamated, or separated by gray clay layers having a thickness of 15–20 cm, are mainly found on the greater reliefs of the Sant'Angelo in Pontano village.

19The study area is located to the east of the Sibillini overthrust and includes the outermost portion of the Marche chain, largely buried under the post-transgressive Plio-Pleistocene succession. The most recent rocks involved in the chain are attributable to the lower Pliocene, they lie in paraconcordant discontinuity or in discordance on the underlying Colombacci Clays of the upper Messinian in the central-northern Marche Region, and in stratigraphic continuity on the turbidites of the Laga Formation in the southern area (Calamita et al., 1991). The most external outcrops of the chain are emerging from the post-transgressive Plio-Pleistocene succession, and are represented by the anticlinal structures of Pesaro-Senigallia, Mount Conero, Polverigi and Porto San Giorgio. Here the compressive deformation is more recent involving also the deposits of the middle-upper Pliocene. The external domain, starting from the upper Messinian of the Colombacci Clays, is gradually more recent towards the east, where the soils of the middle-upper Pliocene are involved in the compression (Calamita et al., 1991). After a phase of subsidence beginning in the Upper Pliocene, the uplift probably began in the lower Pleistocene, which leads in several phases and in a differentiated way to the general emergence of the Quaternary basin (Nanni et al., 1986).

20Magnetic data for the Italian territory taken with onshore station spacing between 1 km and 10 km permitted a 2 km grid cell size for a color shaded relief map of Figure 7A, with a contour interval of 5 nT (Tontini et al., 2004). The geomagnetic anomaly at the Sant'Angelo in Pontano morphology position is around 15-25 nT, where a promontory shape of a geomagnetic increase is observed that exactly coincides with the circular feature. Because of the resolution limits, it can be affirmed that the geomagnetic anomaly variations going from the outside towards the center of the circular morphology are between 5 and 10 nT. Moreover, Italian gravimetric stations were distributed so to obtain a cover of one station for every km2, with a resolution of 10 mGal (APAT Collaboration, 2005). The gravimetric anomaly at the Sant'Angelo in Pontano morphology position resulted negative with a 60-70 mGal intensity, which constitutes a profile descending to the southeast of the Marche Region territory (fig. 7B). No particular concentrated anomalies are observable in the circular morphology within the gravimetric resolution limits.

4.2. Hydrographical features

21Landslides, concentrated mostly near the channels of the main torrents, can be observed collapsing on the sandy, turbiditic rocks. Erosion processes are generally due to water run-off on the predominantly clayey, clay-loamy soils, with formation of gullies. The area is bounded both externally and internally by lower water-bearing moats. Inside the circular structure, water collectors originate from run-off on clay soils, until they flow into the Ete Morto Creek. The morphology corresponds to two secondary basins of the Ete Morto Creek collectors, externally bounded by the studied ridges. The hydrographic lattices of the area have the dendritic shape typical of terrigenous deposits (fig. 6). In particular, the hydrological lattices within the circular feature led to the formation of concentric patterns. In fact, the hydrographic network is almost perfectly circular and is made up of two concentric branches of the Ete Morto Valley. The diameter formed by internal branches is around 1.8 km, while the one formed by external branches is around 5.4 km. The basin area was defined with the help of a topographic map and bounded by the maximum rim altitude. It was calculated to be Sb = 11.9 km2 (fig. 8), and its perimeter Pb = 14.2 km while the mainstream length of Ete Morto basin inside the circular structure was calculated to be Lb = 4.27 km. These values deviate significantly from the Hack Law which produces a Lb = 6.63 km starting from Sb. Basin area and perimeter define a low Gravelius Index Ig = 1.16 and a Ra = 0.91 consistent with a nearly circular morphology.

Fig. 8 - Hydrographic basins of the area with the Fiastra River and the Salino Creek that appear diverging at the structure; main points of the rim are located in Sant’Angelo in Pontano (A), Madonna delle Grazie (B), Contrada Salegnano (C) and the provincial road SP166 (D), chosen for the analysis. The rim altitude profile throughout points A, B, C and D is reported below.
Fig. 8 - Bassins hydrographiques de la zone étudiée avec la rivière Fiastra et le ruisseau Salino qui paraissent divergentes au niveau de la structure ; les principaux points du cercle sont situés à Sant'Angelo in Pontano (A), Madonna delle Grazie (B), Contrada Salegnano (C) et provincial road SP166 (D), choisis pour l'analyse. Le profil d'élévation du trajet à travers les points A, B, C et D est indiqué au-dessous.

Fig. 8 - Hydrographic basins of the area with the Fiastra River and the Salino Creek that appear diverging at the structure; main points of the rim are located in Sant’Angelo in Pontano (A), Madonna delle Grazie (B), Contrada Salegnano (C) and the provincial road SP166 (D), chosen for the analysis. The rim altitude profile throughout points A, B, C and D is reported below.Fig. 8 - Bassins hydrographiques de la zone étudiée avec la rivière Fiastra et le ruisseau Salino qui paraissent divergentes au niveau de la structure ; les principaux points du cercle sont situés à Sant'Angelo in Pontano (A), Madonna delle Grazie (B), Contrada Salegnano (C) et provincial road SP166 (D), choisis pour l'analyse. Le profil d'élévation du trajet à travers les points A, B, C et D est indiqué au-dessous.

4.3. Rim and depression - breaching

22The Regional Technical Map of the Marche Region in 1:10,000 scale was used to estimate the dimensions of the internal rim of the studied landform that were initially based upon the points of greatest altitude of the basin area (fig. 8). Moreover, from satellite images and DEM, a shorter rim was hypothesised for the circular structure with respect to those of the basin, following the shortest path on the highest altitudes. Specifically, the rim on the map was divided into four sections and its altitudes and positions are reported in Table 1. The elevation profiles are described in Figure 8. The rim extends from Sant'Angelo in Pontano, point A (506.8 m asl), towards the south-east, then curving in a north-easterly direction along the Faleriense road up until Madonna delle Grazie, point B (368 m asl). Overall, the distance between A and B is 5,760 m. The rim continues in a north-western direction following the basin margin up to the point C in Contrada Salegnano (314 m asl), with a total distance between A and C of 6,820 m. Then, the rim continues west along the highest ridge leaving the edge of the basin on the right, it crosses the moat (159 m asl) and climbs up to the provincial road SP166 at point D (300 m asl) covering a total distance of 8,910 m. Finally, the rim continues south-west, curving southwards towards point A with a total perimeter length of P = 13,110 m. The highest altitude reaches 507 m asl, while the lowest altitude is 159 m asl. The average altitude is 373 m asl, the maximum difference in height results being 347 m. From the elevation profiles, see Figure 8 bottom, it is evident that the altitudes of the circular feature decrease to the north and north-east in non-regular patterns. In the C-D profile, the rim of the morphology displays erosive features corresponding to deepening of the Ete Morto Creek into the clays. In fact, a low-angle "V" shaped profile has been created with a strike angle of 123°, visible in Figure 2 and Figure 8.

Tab. 1 - Reference points to describe the Sant'Angelo in Pontano structure rim.
Tab. 1 - Points de référence pour décrire le bord de la structure à Sant'Angelo in Pontano.

Tab. 1 - Reference points to describe the Sant'Angelo in Pontano structure rim.Tab. 1 - Points de référence pour décrire le bord de la structure à Sant'Angelo in Pontano.

4.4. Size and shape

23The topography was studied starting from the TINITALY DEM analysis of the Sant'Angelo in Pontano territory. A shaded colored elevation map was calculated together with 50 m spaced contours and displayed in Figure 9. It shows a structure where rims and ditches have been considerably shaped by erosion and landslides. Moderate slopes ranging between 10° to 25° were observed for the annular hills. Steeper slopes up to 40° are found along channels on the western part of the structure where the peaks on the annular ridge are higher, in red of Figure 9. Adjacent basins resulted in having elevations greater than the Ete Morto Creek basin at the same distance from their mouths. In fact, the Fiastra River basin is about 100 m higher than the Ete Morto Creek basin, whereas the Salino Creek basin is about 50 m higher than the Ete Morto Creek basin. N-S and W-E profile sections are shown in Figure 10, together with NW-SE and SW-NE profile sections. Three profile sections describe a dome shaped hill in the central part of the structure delimited by valleys, which altitude of the center is about 418 m. The dome merges with the southern part of the rim in the N-S profile section which has a height of 467 m.

Fig. 9 - Shaded relief DEM map of the selected area with colored altitudes and 50 m elevation lines in blue.
Fig. 9 - Carte MNE de la zone sélectionnée combinant relief ombré et hypsométrie courbes de niveau équidistantes de 50 m en bleu.

Fig. 9 - Shaded relief DEM map of the selected area with colored altitudes and 50 m elevation lines in blue.Fig. 9 - Carte MNE de la zone sélectionnée combinant relief ombré et hypsométrie courbes de niveau équidistantes de 50 m en bleu.

24The maximum diameter between ridges is Dmajor = 4.4 km with a direction of 76°, whereas the minimum diameter is Dminor = 3.1 km. The total internal area has been calculated in S = 10.5 km2, which is slightly smaller than the basin area. A further outer annular ridge is partially visible along the northern part of the morphology, having a curvature radius of about 3.7 km and therefore a presumed average diameter of 7.4 km. The depth to diameter ratio was calculated by the available value for depth by the major axis, which is d = 308 m, and average diameter producing dd = 0.08. Whereas the AR and EL are equal to 0.70 and 0.69 respectively; suggesting that the morphology is symmetric. The index expressing the relationship between the surface S of the morphology and the area of a circle with a perimeter P equal to that of the morphology is IC = 0.77. All the morphological indices relatively to both craters and basins are reported in Table 2.

Fig. 10 - Altimetric profiles extracted from the DEM and amplified by a factor 12.
Fig. 10 - Profils topographiques extraits du MNE et amplifiés d'un facteur 12.

Fig. 10 - Altimetric profiles extracted from the DEM and amplified by a factor 12.Fig. 10 - Profils topographiques extraits du MNE et amplifiés d'un facteur 12.

The four sections N-S, NW-SE, W-E, and SW-NE are indicated by red lines.
Les quatre sections N-S, NO-SE, O-E et SO-NE sont indiquées par des lignes rouges.

Tab. 2 - Crater and hydrographic basin morphological indices.
Tab. 2 - Indices morphologiques du cratère et du bassin hydrographique.

Tab. 2 - Crater and hydrographic basin morphological indices.Tab. 2 - Indices morphologiques du cratère et du bassin hydrographique.

5. Discussion

25In order to describe the morphology and to provide additional constraints regarding its nature and origin, comparisons were made with Earth landforms having similar physical characteristics and dimensions. Since the morphology is located far from the nearest volcanic areas in the Umbria (Stoppa, 1996), and Abruzzo (Barbieri et al., 2002) Regions, 100 km to the west and southwest respectively, it can be excluded that this morphology was of volcanic origin. Moreover, sinkholes and karst dolines are not known to exist in this area (Nisio et al., 2007) and would not be expected to have a rim with an elevation higher than the surrounding topography. Therefore, hypotheses concerning the origin of this morphology might include that of astrobleme, hydrographical formation, a brachyanticline, or the presence of a clay dome, all of which can form in loose sediments and undergo strong erosion processes.

5.1. Impact crater

26In order to reliably suggest that a geomorphological feature is the result of an impact, it is generally necessary to verify that a number of shock-related ejecta characteristics exist in the study area. In fact, a large impact produces pyroclastic flows which travel for tens of km the formed rock is a welded breccia containing tektites and the rock name is suevite. Moreover, characteristics can include (French, 1998): (a) tektites, ejecta or pseudotachylite, (b) radial and ring faulting or uplift, (c) shock textures, shatter cones and high-pressure minerals, (d) high concentrations of Fe/Ni/S and Pl group elements, or other geochemical signatures, (e) circular geophysical anomalies. This preliminary study does not consider characteristics (a), (c) and (d), because physico-chemical soil surveys were not yet carried out in this restricted territory. For what concerns geophysical surveys (e), aeromagnetic and gravimetric anomalies were detected in all the Italian Regions, which are reported in Figure 7, while seismic profiles have not been realized in this particular area. The aeromagnetic map of Figure 7A shows a particular positive anomaly of the magnetic field of about 10 nT, within the sensibility limits of that survey, which is located in correspondence to the morphology. However, confirmed impact craters, of dimensions less than 10 km, correspond to magnetic lows (Grieve and Pilkington, 1996). Therefore these data are not in agreement with an impact origin for the Sant'Angelo in Pontano morphology. Furthermore, impact craters with diameters less than 10 km are often associated with negative gravity anomalies, which can indicate the presence of breccias and sediments of low density (French and Koeberl, 2010). Figure 7B shows gravimetric anomalies with resolutions which are on the limit of those usually associated with small impact craters on the Earth (Grieve and Pilkington, 1996), but no circular anomaly coincides with the position of the studied pattern.

27The composition and material properties of the impacted region, as well as the subsequent geological history of the impact site are relevant for determining the characteristics of a morphology. The stratification of the area struck by an impactor should also give evidence of a characteristic deformation with uplift in the crater rim and dips toward the edges of the crater. Several types of impact craters could be considered from a morphological point of view (Grieve, 1998): (i) simple bowl-shaped craters that are circular depressions with a depth/diameter ratio of roughly 1/5, (ii) complex structures that have much smaller depth/diameter ratios, are larger and their shapes are not as uniform as in the case of simple craters. Additionally, there is often an uplift in the centre of the flat crater floor. Moreover, normal faults and terraced rims were observed in concentric geometries (Melosh, 1989).

28The Sant'Angelo in Pontano circular pattern better matches with morphological cases of complex structures having dd = 0.08 and presents partial evidence of an annular ring. A central uplift may also be present even if it is not surrounded by a flat floor: the inferred uplift is connected in a continuous way to the main ring in the southern direction. The lowest point of the structure, 159 m asl, has a lower elevation than the surrounding basins, 300 m towards west and 200 m towards east. The Intra ring depression is at nearly the same elevation as the terrain outside the circular feature. Moreover, a comparison of terrestrial craters evidenced that the amount of structural uplift hsu is quantitatively related to the final crater diameter D by (Melosh, 1999):

29producing hsu = 257 m above the minimum depth, which corresponds to 416 m asl, about the same than the Sant'Angelo in Pontano central uplift altitude equal to 418 m. Concerning the diameter of the uplift, on all the terrestrial planets it is roughly 22% of the final rim-to-rim crater diameter (Melosh, 1999), that in this case corresponds to 825 m, much less than that observed here. Because much of the original rim has slid into the crater bowl given that the wall undergoes collapses, complex craters appear larger than the transient crater from which they form (Melosh, 1989). A useful scaling quantitative association between the rim to rim diameter of a complex crater and the transient crater Dt diameter is

30where Ds-c = 3.2 km is the transition diameter from simple to complex for craters on earth.

31Expression [9] suggests a Dt = 3.2 km for the morphology. According to Melosh (1989), elliptical craters can result from either low-velocity (0.5–10 km s-1) and moderately oblique impacts with incidence angles of 30° to 60°, or from higher-velocity extremely oblique impacts with incidence angles less than 10°. Regarding the former impact, an elliptical landform like that observed in Sant'Angelo in Pontano with Dt = 3.2 km diameter, scaling laws require that there might have been an impactor having a 100 – 300 m diameter (Melosh, 1989). If this occurred, given the supposed low-velocity, the bulk of the meteorite responsible for the crater might not have completely disintegrated. Moreover, a relatively low dd ratio in soft-clay ground might further support the hypothesis that it was a wet target (Kenkmann et al., 2007). The section provided by the Regional Survey Map n. 314 of Montegiorgio (fig. 4), indicates irregular strata sloping eastwards towards the Adriatic Sea and westwards towards the Appennine Chain; therefore presenting divergent dips. However, the section does not provide a sufficiently fractured crust which should be expected for an impact crater. Furthermore, identical features can be produced by numerous other endogenic geological processes such as diapirism (French and Koeberl, 2010).

5.2. Hydrographical basin

32The erosion processes and depositional products of water on Earth are so active that craters are among the rarest landforms (Melosh, 1999). The Sant'Angelo in Pontano morphology is an example of erosionally shaped landforms. The dominant erosion processes that determined the morphology must have been hydrological processes. In fact, all the hilly soft soil of the Marche Region is furrowed by rivers and creeks generally flowing down from the Appennine Chain, running parallel towards the Adriatic coast (E-NE).

33The question is: could a hydrological process have been able to form a such rounded hydrographical basin? To respond it can be observed that the hydrography of the area (fig. 6), shows the Fiastra River and the Salino Creek descending from the Appennine Chain, running parallel towards the Adriatic coast west of Sant'Angelo in Pontano. However, in the Gualdo territory, they suddenly diverge, the former going north and merging with the Chienti River, and the latter going south-east and merging with the Tenna River. Further evidence of the turn to the north of the Fiastra river lies in the observation of a saddle on the external ring, see Figures 2 and 9, to be interpreted as an erosion surface that could represent the ancient course of the Fiastra River (Dramis et al., 1992), which continues eastwards along the Tifo Valley and the Ete Morto Creek. This sudden divergence suggests the presence of an obstacle in the river basins which was formed higher than the adjacent basins elevations in correspondence of Sant'Angelo in Pontano. Indeed, a secondary hydrographical network has been created within the obstacle. In fact, this would explain the existence of the Ete Morto Creek.

34The hydrographical network is concentric inside the entire structure. From the point of view of the hydrological basin quantitative morphology the mainstream length Lb inside the circular feature does not seem to satisfy Hack's law: the expected mainstream length should be significantly greater than it is. Or else starting with the mainstream length Lb = 4.27 km, the required basin area following Hack's law should be Sb = 5.7 km2 which is about half that of the observed basin area. That means that the expected basin area should have been more elongated and less circular.

5.3. Brachyanticline

35The Sant'Angelo in Pontano structure can be regarded as a dome - brachyanticline, that is an anticline formed by compression along two directions. It is classified according to the length/width ratio and has the characteristic of a length that is approximately double of the width. The brachyanticline is formed during the compressive tectonic phases and the axis of the fold takes a transverse direction with respect to the direction of compression, therefore geometrically tends to have elliptic major axes perpendicular to the thrust. The morphology of Sant’Angelo in Pontano shows in Figure 8 a slight elongation in the SW-NE direction and coinciding with the direction of regional compression. This is an opposite trend with respect to the directions of a brachyanticline, from which we would expect the elongation to be directed towards NW-SE like the anticline near the village of Sant'Angelo in Pontano with NW-SE axis delimited by two high-angle faults. Examples of brachyanticlines in the Marche region are suggested for the northernmost part of the region (Nesci et al., 2002; Nesci et al., 2005), and have the elliptic major axes in the NW-SE direction. Finally, some examples of the brachyanticlinal structures tend to contours positive gravity and geomagnetic anomalies (Anikeyev et al., 2020), including the suggested Monte Conero brachyanticline in the Marche region (Mussi et al., 2016), see Figure 7. Although a slight positive geomagnetic anomaly is observed at the morphology of Sant'Angelo in Pontano, see Figure 7A, the orientation of the structure and gravitational anomalies do not seem to support the brachyanticline hypothesis.

5.4. Clay diapir

36The presence of a dome-like central elevation could have been the origin of the circular morphology due to fluvial erosion. In other words, the hydrological erosion of the dome could have produced annular rings as they are seen from space. Several studies have shown that this is possible. Among them, a study of the Clay Creek dome (McDowell, 1951), revealed an almost perfectly circular structure with dimensions similar to that of Sant'Angelo in Pontano, about 4.5 km in diameter. The formation of the Clay Creek dome was explained by upward intrusion of salt that lifted the strata above active diapirs above the regional elevations of the coeval strata in the surrounding areas (McDowell, 1951). In fact, the crest of Clay Creek dome is approximately 800 m higher than the surrounding areas. Moreover, normal faults, which are radially distributed, are also common in the overlying strata above active domes (Yin et al., 2006). These are usually located inside the arched area of the overlying strata, increasing toward the crest and going to zero at the edge of the dome. Three major normal faults are supposed to maintain a constant volume (Yin et al., 2006), allowing for a graben to be created above the dome. Faults represented in past works (Micarelli et al., 2014) are shown in Figure 3 and form a similar pattern of converging faults, although they are not exactly those required by Yin et al. (2006), they may be an incomplete representation of those actually existing in the structure. Accordingly, the circular central hole of the Sant'Angelo in Pontano morphology, excavated by erosion, does not preclude an erosion process started from the graben and maintained in a circular shape due to converging faults.

37The section reported by the Regional Survey Map n. 314 of Montegiorgio which, as observed above, indicates irregular eastward dip of the strata towards the Adriatic Sea and westward dip towards the Appennine Chain, is compatible with a diapir rising and lifting the sedimentary pile of the area. On the other hand, neither salt diapirs have ever been observed in this region of Italy, nor traces of salt diapir have been found inside the annular landform, which is deeply dug. Instead, mud extrusion producing pools and small domes is a widespread phenomenon at distances from a few km to tens of km from the center of the morphology (Farabollini et al., 2005; Farabollini and Scalella, 2014), and a case is known in this area along the Faverchiara Valley (fig. 8). In fact, it is also a well-documented phenomenon in the Marche (Bonasera, 1952; Martinelli, 1999; Maestrelli et al., 2017) and Abruzzo (Rainone et al., 2015) Regions, whereas it has been widely investigated in the Emilia-Romagna Region (Manga and Bonini, 2012). Mud extrusion phenomena are mainly reported in areas where thrust fronts are observed, mostly in Eastern Italy (Martinelli and Judd, 2004). Most of them are small (up 3 m high and 20 m large) formations called mud pools, salsas and gryphons, which build-up clay domes. These domes are currently too small to hypothesise that the morphology is of volcanic mud origin. Mud volcanoes develop when fine-grained sediments ascend within a lithological section because of their buoyancy; they are seldom associated to diapirism (Kopf, 2002).

38Moreover, in some studies it has been reported that the elongation of a mud volcano could be linked to the stress orientation of the region (Bonini and Mazzarini, 2010). Furthermore, the tectonic evolution of this area, where the Appennine Chain is responsible of a thrust stress SW-NE, is able to generate a hydrostatic over pressure allowing the rise of clays and fluids. It led to the generation of the aforementioned mud volcanoes on a small scale (Martinelli and Judd, 2004). The direction of the elongation of the Sant'Angelo in Pontano morphology appears in agreement with the stress orientation of the area, but no evidence has been found that leads to the hypothesis that it is a large eroded mud volcano. Then we can only hypothesize a deep diapir may have pushed the surface layers upwards (Kopf, 2002) creating the anticline in the clays as shown in Figure 11C. A Y-pattern dome initiates with three major normal faults Yin and Groshong (2006). Being raised by a diapir, the overlying clay layers are fractured in a graben, as shown for the model proposed by Yin and Groshong (2007). Unlike this model, here the upward thrust would be generated by a clay diapir instead of salt. Model’s figure of Yin and Groshong (2007) is rotated by 10° in Figure 11B so as to mimic the faults (fig. 3) reported in Figure 11A. Given that the faults reported in past geological studies are two (Micarelli et al., 2014), an additional fault was hypothesized to close the graben inside the structure. However, the Y-pattern is not at the center of the displayed circles in Figures, which can be a consequence of erosion due to the Salino Creek and the Tenna River at south of the landform. The edges of the graben are prone to erosion. Rain can quickly erode the dome flanks, with channels coming out radially, and the central graben may have been deeply grooved due to erosion, forming gullies. The pressure of the clay-rich mud could inject the fluid into the fractures, and in some cases it reached the surface generating mud volcanoes (fig. 11C).

Fig. 11 - An illustration of the hypothesized structure starting from already reported (continuous red lines) and hypothesized (dashed red lines) faults (A); the Y-graben model of the dome, proposed by Yin and Groshong (2007), formed by the upward migrating mass of sediments (B); mud volcanoes occurred where the muddy sediments reached the surface in a section AA' of the graben (C).
Fig. 11 - Une illustration de la structure hypothétique à partir des failles déjà signalées (traits rouges pleins) et hypothétique (traits rouges discontinus) (A) ; le modèle Y-graben du diapir, proposé par Yin et Groshong (2007), formé par la masse de sédiments migrant vers la surface (B) ; des volcans de boue se forment là où des sédiments boueux ont atteint la surface, illustré dans une section AA' du graben (C).

Fig. 11 - An illustration of the hypothesized structure starting from already reported (continuous red lines) and hypothesized (dashed red lines) faults (A); the Y-graben model of the dome, proposed by Yin and Groshong (2007), formed by the upward migrating mass of sediments (B); mud volcanoes occurred where the muddy sediments reached the surface in a section AA' of the graben (C).Fig. 11 - Une illustration de la structure hypothétique à partir des failles déjà signalées (traits rouges pleins) et hypothétique (traits rouges discontinus) (A) ; le modèle Y-graben du diapir, proposé par Yin et Groshong (2007), formé par la masse de sédiments migrant vers la surface (B) ; des volcans de boue se forment là où des sédiments boueux ont atteint la surface, illustré dans une section AA' du graben (C).

6. Conclusions

39In a hilly area of the Marche Region in Central Italy, where field reconnaissance is sometimes difficult due to the highly eroded terrain, data obtained from space imaging have been very useful for identifying circular patterns. The morphology of this study, located in Sant'Angelo in Pontano Village, is clearly visible on satellite images and is composed of annular circular valley and hill system. No other similar landforms have been recorded throughout the entire Italian Peninsular:

  • It is located in the Messinian foredeep deposits of the Central Apennines, with a rim diameter of 3.75 km and a central uplift connected to southern part of the rim.

  • As the Sant'Angelo in Pontano morphology was formed in clays of the Lower Pliocene and clays are believed to have emerged definitively after the Upper Pliocene, its age can be constrained to the Lower Pleistocene < 2.5 My.

  • Karst depressions, large dissolution sinkholes, and glacial erosions have elevations lower than the surrounding topography and are also absent in this area.

  • The hydrographic organization inside the structure is concentric. Usually, it is only possible to find such a concentric trend in impact craters, sedimentary domes, and volcanic landforms. However, salt domes and magmatic activity are not found in this region. Furthermore, an igneous volcanic origin can be excluded as there are no volcanic lithologies but rather sedimentary ones in all the surrounding areas of the Marche Region.

  • The first hypothesis which was investigated in this study was that of an ancient impact crater, with a transitory diameter of 3.2 km, which might suggest a possible formation by a low velocity, moderate angle, impactor of 100–300 m in diameter. Although the central uplift satisfies the relation (8) with the final crater diameter, the uplift diameter does not seem to be comparable to that of a crater on the Earth. Intense fracturation and shock-related eject characteristics are not evident. Furthermore, geomagnetic and gravimetric data are not in agreement with an impact origin.

  • According to both qualitative and quantitative morphological analyses, there are few evidence of an exclusively hydrological origin of this basin.

  • The hypothesis of a brachyanticline was also investigated. Indeed, the elongation of the structure in disagreement with the tectonic observations, instead of what was confirmed by some examples of brachyanticlinals suggested for the northernmost part of the Marche Region.

  • The Y-pattern supposed for salt dome examples was hypothesized at the origin the Sant'Angelo in Pontano landform as a consequence of a clay diapir. The Y-pattern followed by erosion is in a better agreement with past observations and the quantitative analysis of the hydrography.

40This last hypothesis was suggested by a particular endogenic phenomenon of mud extrusion which can be presently observed in this area; it can be often associated with clayey diapirism. The elongation of the dome is oriented with the same direction as the tectonic stress of this area, as observed in some mud volcanoes. Rock samples analyses are necessary to completely exclude the astrobleme hypothesis. Geostructural field surveys would be necessary to support the hypothesized Y-pattern fault model.

*Corresponging author: Tel: +39 (0)759 27 11 50
sabrinacolucci1969@gmail.com (S. Colucci)

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Annexe

Version française abrégée

La morphologie générale de la zone sud du bassin des Marches est caractérisée par des collines douces et des vallées qui descendent des Apennins vers la mer Adriatique. Les photos analysées dans ce travail sont des images optiques satellitaires, montrant un dispositif morphologique circulaire situé à environ 35 km à l'ouest de la côte Adriatique et à 15 km à l'est de la chaîne des Apennins, avec pour coordonnées 43°06'N et 13°25'E (fig. 1). Le bassin comporte deux cercles concentriques, mis en évidence par les différents tons de vert dus à la végétation plus riche des dépressions, et par le vert et le brun des plateaux cultivés. Deux dépressions concentriques sont situées à environ 0,9 km et 2,7 km du centre de la structure. Les altitudes les plus élevées des collines de la partie ouest dépassent 500 m dans la commune de Sant'Angelo in Pontano, tandis que les altitudes les plus basses des collines n'atteignent que 300 m vers le nord-est de la structure de la Contrada San Martino (fig. 2). Le diamètre du bord de ces collines est en moyenne de 3,75 km. La dépression annulaire a été creusée par les ruisseaux Tifo et Ete Morto, dont l'altitude minimale est de 159 m, alors qu'elle est délimitée par la rivière Fiastra dans la partie ouest et le ruisseau Salino dans la partie sud. Les collines au-delà de la deuxième dépression circulaire, observables exclusivement dans les parties nord et nord-est de la morphologie (fig. 2) forment une deuxième crête annulaire qui a des hauteurs comprises entre 300 et 400 m. La marge de ces collines extérieures est en moyenne à 3,7 km du centre de la morphologie. La superficie totale entourée par le cercle en pointillé de la Figure 1, y compris l'anneau extérieur, est estimée à environ 50 km2.

Les lithologies caractéristiques de cette zone font partie du bassin de la Laga, le plus grand des « petits bassins » de l'Ombrie-Marche. La zone de Fermo, contenant la forme en question, est la zone la plus déprimée du bassin de la Laga ; elle comporte une succession de dépôts argileux profonds de faciès bathyal remontant au Messinien. Les Figures 3 et 4 montrent un pli anticlinal près du village de Sant'Angelo in Pontano avec un axe NO-SE délimité par deux failles inverses à grand angle. Ces failles déterminent le mouvement du membre évaporitique sur le post-évaporitique récent et celui du membre post-évaporitique de la Laga sur les argiles bleues. En continuant vers le NE, on trouve un anticlinal dans la formation Colombacci (fig. 5A). Au nord de la morphologie, il y a davantage de failles hypothétiques avec des directions SO-NE (fig. 3). Des glissements de terrain, concentrés principalement à proximité des vallées des principaux torrents, peuvent être observés dans des terrains sableux et au faciès turbiditiques. Les processus d'érosion sont généralement dus au ruissellement de l'eau sur les sols à prédominance argileuse, argilo-limoneuse, avec formation de ravines. La zone est délimitée par les chenaux à l'extérieur et à l'intérieur, les chenaux plus profonds contenant de l'eau. À l'intérieur de la structure, des collecteurs d'eau se constituent sur les sols argileux, dus au lavage et au ruissellement, jusqu'à ce qu'ils se jettent dans le ruisseau Ete Morto. Le pic central de la morphologie est situé entre deux bassins secondaires des collecteurs du ruisseau Ete Morto, ces bassins secondaires sont délimités extérieurement par les crêtes étudiées. Le réseau hydrographique de la zone a la forme dendritique typique des dépôts terrigènes énumérés ci-dessus (fig. 6).

L'anomalie géomagnétique correspondant à la position de la morphologie de Sant'Angelo in Pontano est d'environ 15-25 nT. On y observe une forme promontoire d'augmentation géomagnétique qui coïncide exactement avec la morphologie (fig. 7). Du fait des limites de résolution, on peut affirmer que les variations d'anomalie géomagnétique allant de l'extérieur vers le centre de la morphologie circulaire sont comprises entre 5 et 10 nT. De plus, l'anomalie gravimétrique à la position de la morphologie de Sant'Angelo in Pontano est négative avec une intensité de 60 à 70 mGal, qui a ici une intensitè décroissante dans la direction sud-est. Aucune anomalie concentrée particulière n'est observable dans la morphologie circulaire dans les limites de résolution gravimétrique.

Le bord de la carte dans la Figure 8 a été divisé en quatre sections pour estimer les dimensions du bord interne de la morphologie, et les altitudes et positions sont indiquées (tab. 1). Les profils topographiques sont décrits au bas de la Figure 8 ; la longueur totale du périmètre est de 13 110 m. L'altitude la plus élevée est de 507 m, tandis que l'altitude la plus basse est de 159 m. L'altitude moyenne est de 373 m au-dessus du niveau de la mer, le dénivelé maximal étant de 347 m. D'après les profils topographiques (fig. 8) il est évident que les altitudes de la structure annulaire diminuent vers le nord et le nord-est de façon irrégulière. Dans le profil C-D, le bord de la morphologie est coupé par un processus érosif qui approfondit le ruisseau Ete Morto dans les argiles. Par consequent, le profil en «V» où la crête atteint l'altitude minimale a été créé avec un angle de 123° (fig. 2, 8). La topographie a été étudiée à partir de l'analyse par TINITALY DEM (fig. 9). Les sections de profil N-S et O-E sont illustrées ainsi que les sections de profil NO-SE et SO-NE (fig. 10). Elles montrent une colline en forme de dôme dans la partie centrale de la structure, délimitée par des dépressions ; l'altitude du centre est d'environ 418 m. Le dôme se confond avec la partie sud de l'anneau qui a une hauteur de 467 m, section N-S. Le diamètre maximum de la crête annulaire est de 4,4 km avec une direction de 76°, alors que le diamètre minimum est de 3,1 km. La superficie intérieure totale a été évaluée à 10,5 km2, ce qui est légèrement plus petit que la superficie du bassin du ruisseau Ete Morto. Une autre arête annulaire extérieure est partiellement visible le long de la partie nord de la morphologie, ayant un rayon de courbure d'environ 3,7 km et donc un diamètre moyen présumé de 7,4 km. Les indices morphologiques relatifs au cratère et aux bassins (tab. 2) suggérent que la morphologie est elliptique.

L’étude conclut que comme la structure de Sant'Angelo in Pontano s'est formée dans des argiles du Pléiocène inférieur et que son âge peut être limité à < 2.5 Ma. Les dépressions karstiques, les grandes dolines de dissolution et les forms d'érosion glaciaire possèdent des altitudes inférieures à celles du relief environnant et sont également absentes dans cette zone. Bien que l'hydrologie ait joué un rôle fondamental dans la formation de la morphologie de Sant'Angelo in Pontano, l'analyse morphologique qualitative et quantitative suggère la concomitance d'un autre facteur dans la formation de tels anneaux circulaires. L’organisation hydrographique de la structure est de type concentrique. De tels dispositifs concentriques sont généralement propres aux cratères d'impact, aux dômes sédimentaires et aux formes volcaniques. Or les dômes de sel et les marques d'activité magmatique ne peuvent être trouvés dans cette région. De plus, une origine volcanique peut être exclue car il n'y a que des lithologies sédimentaires dans toutes les zones environnantes de la région des Marches. La première hypothèse qui a été examinée dans cette étude concernait un ancien cratère d'impact ou astroblème, avec un diamètre de la cavité transitoire de 3,2 km, ce qui suggérait une vitesse d’impact faible, sous un angle modéré, avec un impacteur de 100 à 300 m de diamètre. Bien que la hauteur du pic central corresponde à celle des pics centraux des cratères terrestres de même diamètre, le diamètre du dôme ne correspond pas à ceux caractéristiques des cratère sur Terre. Les caractéristiques d'une structure d’impact, avec une fracturation intense et la présence d’ejecta ne sont pas évidentes ici. De plus, les données géomagnétiques et gravimétriques ne sont pas en accord avec une origine impactique. L'hypothèse d'un brachyanticlinal a également été étudiée qui a montré un allongement de la structure en désaccord avec les observations tectoniques. Un phénomène endogène particulier d'extrusion de boue peut être actuellement observé dans cette zone ; il peut être souvent associé à un diapirisme argileux. Par conséquent, une hypothèse secondaire examinée dans cette étude concernait une origine diapirique argileuse suivie d'une phase d'érosion. Le modèle graben en Y proposé pour le centre des diapirs argileux semble mieux concorder avec les résultats de l'analyse quantitative du bassin hydrographique de Sant'Angelo in Pontano, qui suggère que la morphologie circulaire peut avoir été-façonnée par l'érosion.

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Table des illustrations

Titre Fig. 1 - Google Earth image of Marche Region, Central Italy (A), delimited by a dashed line, where the structure was observed (B), 10X magnification of the structure; dashed circle (C): outer limit of the whole annular landform.Fig. 1 - Image Google Earth de la région des Marches, au centre de l'Italie (A) où la structure a été observée (B), agrandissement 10X de la structure, délimitée par un cercle en pointillé (C).
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-1.png
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Titre Fig. 2 - Google Earth image with elevation 3X showing two annular rings viewed in 245°N direction from a height of 2.7 km; colored altitude map in the upper left; Contrada San Martino is the elongated central hill that forms the north ridge of the inner ring; the Apennine chain is at the top background.Fig. 2 - Image Google Earth avec élévation 3X, montrant deux anneaux vus vers 245 ° N d'une hauteur de 2,7 km ; carte hypsométrique colorée en haut à gauche ; Contrada San Martino est la colline centrale allongée qui forme la crête nord de l'anneau intérieur ; la chaîne des Apennins est en arrière-plan au-dessus.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-2.png
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Titre Fig. 3 - Structural map of the area covered by the annular landform; main geological formations of the Laga Basin area: from Ricci Lucchi et al. (1975) and Micarelli et al. (2014); a circular dashed line surrounds the entire morphology.Fig. 3 - Carte structurale de la zone couverte par la structure annulaire. Principales formations géologiques de la région du bassin de Laga : d’après Ricci Lucchi et al. (1975) et Micarelli et al. (2014) ; une ligne en pointillé entoure toute la structure.
Légende 1. Laga Basin. Blue Clays; 2. Monte dell'Ascenzione Member; 3. Spugnone Member; 4. Pelitic; 5. Colombacci Formation. Laga Formation: 6. Postevaporitic Member; 7. Evaporitic Member. 8. Camerino Basin; 9. Internal Marche Basin; 10. External Marche Basin; 11. Fault; 12. Anticlinal; 13. Synclinal; 14. Thrust.1. Bassin de Laga. Argiles Bleues ; 2. Membre du Monte dell'Ascensione ; 3. Membre du Spugnone ; 4. Pélitique ; 5. Formation à Colombacci. Formation de la Laga : 6. Membre Postvaporitique ; 7. Membre Évaporitique ; 8. Bassin de Camerino ; 9. Bassin Interne des Marches ; 10. Bassin Extérieur des Marches ; 11. Défauts ; 12. Anticlinal ; 13. Synclinal ; 14. Poussée.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-3.png
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Titre Fig. 4 - Stratigraphic details of section A-A (fig. 3) across the annular structure; adapted from the Marche Geological Survey, scale 1:50000, at www.isprambiente.it. LAG – Laga Formation, Miocene sup., Messinian; FCO – Colombacci formation, Miocene sup., Messinian; FAA – Clays formation, Pliocene inf., Pleistocene sup.Fig. 4 - Détails stratigraphiques de la section A-A (Fig. 3) à travers la structure annulaire ; adaptée de Marche Geological Survey, échelle 1:50000, sur www.isprambiente.gov.it. LAG – Laga Formation, Miocene sup., Messinian ; FCO – Colombacci formation, Miocene sup., Messinian ; FAA – Clays formation, Pliocene inf., Pleistocene sup.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-4.png
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Titre Fig. 5 - Two photos taken from the centre of the morphology, one to the east (A) and the other to the west (B); the points of view are indicated by the respective colored altitude maps.Fig. 5 - Deux photos prises depuis le centre de la morphologie, l'une à l'est (A) et l'autre à l'ouest (B) ; les points de vue sont indiqués par les cartes d'élévation colorée respectives.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-5.png
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Titre Fig. 6 - Hydrography of the study area at a scale of 1:100,000 by Geoportale Nazionale. Ministero dell'Ambiente at http://www.pcn.minambiente.it/​mattm/​ (2011).Fig.6 - Hydrographie de la zone d’étude à l'échelle 1:100000 par Geoportale Nazionale. Ministero dell'Ambiente à http://www.pcn.minambiente.it/​mattm/​ (2011).
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Titre Fig. 7 - Sea level total magnetic field anomaly map Fig. 7 - Carte des anomalies du champ magnétique total au niveau de la mer
Légende (A), from Chiappini et al. (2000), and gravity anomaly map with three provinces (B), downloaded at www.isprambiente.gov/Media/milione_grav/milionegrav_2004/milione.html, both for the region of Central Italy where the ring structure is inside the dashed circle.(A), tirée de Chiappini et al. (2000), et carte des anomalies gravimétriques avec trois provinces (B), téléchargées sur www.isprambiente.gov/Media/milione_grav/milionegrav_2004/milione.html, toutes deux pour la région d'Italie centrale où la structure annulaire est à l'intérieur du cercle en pointillé.
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Titre Fig. 8 - Hydrographic basins of the area with the Fiastra River and the Salino Creek that appear diverging at the structure; main points of the rim are located in Sant’Angelo in Pontano (A), Madonna delle Grazie (B), Contrada Salegnano (C) and the provincial road SP166 (D), chosen for the analysis. The rim altitude profile throughout points A, B, C and D is reported below.Fig. 8 - Bassins hydrographiques de la zone étudiée avec la rivière Fiastra et le ruisseau Salino qui paraissent divergentes au niveau de la structure ; les principaux points du cercle sont situés à Sant'Angelo in Pontano (A), Madonna delle Grazie (B), Contrada Salegnano (C) et provincial road SP166 (D), choisis pour l'analyse. Le profil d'élévation du trajet à travers les points A, B, C et D est indiqué au-dessous.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-15.png
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Titre Tab. 1 - Reference points to describe the Sant'Angelo in Pontano structure rim.Tab. 1 - Points de référence pour décrire le bord de la structure à Sant'Angelo in Pontano.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-16.jpg
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Titre Fig. 9 - Shaded relief DEM map of the selected area with colored altitudes and 50 m elevation lines in blue.Fig. 9 - Carte MNE de la zone sélectionnée combinant relief ombré et hypsométrie courbes de niveau équidistantes de 50 m en bleu.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-17.png
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Titre Fig. 10 - Altimetric profiles extracted from the DEM and amplified by a factor 12.Fig. 10 - Profils topographiques extraits du MNE et amplifiés d'un facteur 12.
Légende The four sections N-S, NW-SE, W-E, and SW-NE are indicated by red lines.Les quatre sections N-S, NO-SE, O-E et SO-NE sont indiquées par des lignes rouges.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-18.png
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Titre Tab. 2 - Crater and hydrographic basin morphological indices.Tab. 2 - Indices morphologiques du cratère et du bassin hydrographique.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-19.jpg
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Titre Fig. 11 - An illustration of the hypothesized structure starting from already reported (continuous red lines) and hypothesized (dashed red lines) faults (A); the Y-graben model of the dome, proposed by Yin and Groshong (2007), formed by the upward migrating mass of sediments (B); mud volcanoes occurred where the muddy sediments reached the surface in a section AA' of the graben (C).Fig. 11 - Une illustration de la structure hypothétique à partir des failles déjà signalées (traits rouges pleins) et hypothétique (traits rouges discontinus) (A) ; le modèle Y-graben du diapir, proposé par Yin et Groshong (2007), formé par la masse de sédiments migrant vers la surface (B) ; des volcans de boue se forment là où des sédiments boueux ont atteint la surface, illustré dans une section AA' du graben (C).
URL http://journals.openedition.org/geomorphologie/docannexe/image/17007/img-22.png
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Référence électronique

Sabrina Colucci et Cristiano Fidani, « Preliminary geomorphological and hydrographical characterization of a circular structure in the Marche Region (Central Italy) and its possible origin », Géomorphologie : relief, processus, environnement [En ligne], vol. 28 - n° 2 | 2022, mis en ligne le 05 juillet 2022, consulté le 12 août 2022. URL : http://journals.openedition.org/geomorphologie/17007 ; DOI : https://doi.org/10.4000/geomorphologie.17007

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Auteurs

Sabrina Colucci

Geo & Geo, Geological Office Faraone, 66026 Ortona, Italy

Cristiano Fidani

Osservatorio Sismico “Andrea Bina”, 06121 Perugia, Italy

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© Groupe français de géomorphologie

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