Geology of the eastern slopes of the Simbruini Mts. between Verrecchie and Capistrello (Central Apennines – Abruzzo, Italy)

ABSTRACT This paper presents the results of a geological mapping project across the eastern Simbruini Mts. and the higher Roveto Valley (Central Apennines). The study area belongs to the Latium-Abruzzi Domain, which is characterized by a Cretaceous and Miocene carbonate platform succession followed by a thick upper Miocene terrigenous succession. A unique feature of the study area is the existence of a thick clastic unit, the ‘brecce della Renga fm.’; this unit was produced by the dismantling of the margins of a large pre-orogenic extensional structural high, which rose within the foredeep basin starting in the early Tortonian. Following the Messinian-Pliocene Apennine chain building phase, the area was subjected to post-orogenic Quaternary extension, related to the opening of the Tyrrhenian sea. A geological map on the 1:20,000 scale illustrates the main stratigraphic and structural features of the area.


Introduction
The present paper is supported by a geological map (Main Map) of a sector of the Simbruini Mountains and the higher Roveto Valley (Central Apennines), in the Latium-Abruzzi geological domain (Figure 1). This is the second of two companion geological maps of the Simbruini Mts., the first (northern Simbruini Mts.) having already been published (Fabbi, 2016). Due to this, obvious similarities occur in the text, concerning the stratigraphic description and the geological setting. The new map covers an area roughly oriented NW-SE and about 70 km² wide, located in the westernmost sector of the Abruzzo region (Province of L'Aquila, Central Italy). The western part of the study area falls within sheets #367 'Tagliacozzo' and #376 'Subiaco' of the official Geological map of Italy on the 1:50,000 scale (Servizio Geologico d'Italia, 1998, 2005, while the eastern part would fall in the sheet #377 'Trasacco' whose realization is still not planned.
The map was conceived originally as a byproduct of a research project whose main target was to investigate the sedimentology of the 'brecce della Renga fm.' (Fabbi & Rossi, 2014see below) and the paleostructural features of the late Miocene Simbruini structural high (Carminati, Fabbi, & Santantonio, 2014;Fabbi, Galluzzo, Pichezzi, & Santantonio, 2014see below). The main differences with the already published geological cartography (Devoto, 1970;Servizio Geologico d'Italia, 1998, 2005 of the area are due to the higher detail of surveying which allowed the accurate mapping of each different lithofacies of the 'brecce della Renga fm.', a more detailed representation of the Mesozoic substrate and a reinterpretation of structural features, that were misrepresented or erroneously interpreted in the previous maps.

Regional geological setting
The Central Apennines ( Figure 1) is a mountain chain advancing from SW to NE, whose growth and migration is related, since the Miocene, to the Wdirected subduction of the Adriatic continental crust under the Italian peninsula (Carminati & Doglioni, 2012;Cosentino, Cipollari, Marsili, & Scrocca, 2010 and references therein).
Stratigraphic units represented in the map have been determined using a lithostratigraphic criterion. The lithostratigraphic study was accompanied by biostratigraphic analysis performed both on microfossil assemblages (essentially benthic forams and calcareous algae in thin section) and nannofossils (on smear slides); the determination of macrofossils (e.g. rudist assemblages) resulted also useful for stratigraphic considerations. In addition, the biostratigraphic study has been integrated with sedimentological analysis for the terrigenous deposits (see Fabbi & Rossi, 2014). No formalized stratigraphic units exist for the carbonate platform succession of the central Apennines, so the stratigraphic units used in this paper are the same described in Compagnoni et al. (2005) and Damiani, Catenacci, Molinari, Panseri, and Tilia (1998), with minor differences discussed in the text.

Stratigraphy and geological setting of the mapped area
The stratigraphy of the study area ( Figure 2) essentially reflects the evolution of the Latium-Abruzzi carbonate platform, which hosted shallow water sedimentation from the Late Triassic to the middle Miocene (Accordi & Carbone, 1988;|Chiocchini et al., 2008;Damiani, 1990;Damiani et al., 1998;D'argenio, 1974;Parotto & Praturlon, 1975, 2004. Since the late Miocene the area was involved in the Apennine Chain building (see below) and the sedimentation style changed from carbonatic to lithoclastic/siliciclastic, and finally, following the emersion from the sea and the uplift of the chain in the latest Messinian-Pliocene (Fabbi & Santantonio, 2018), a further shift occurred from marine to continental sedimentation.

Carbonate platform succession
While carbonate sedimentation in the region started in the late Triassic (Chiocchini et al., 2008 and references therein), the earliest part of this sedimentary history, up to the middle Jurassic, crops out outside the mapped area.
The 'Campbelliella limestone' (Figure 3) overlies the 'Saifar limestone', and is a gray-hazel packstone/wackestone, often dolomitized, organized in dm-to m-thick beds, characterized by the abundance of the dasycladal alga Campbelliella striata, along with Salpingoporella annulata, Clypeina solkani, C. radici, Favreina salevensis, Arenobulimina sp., Triploporella neocomiensis, gastropods, ostracods and bioclastic debris. This unit has been separated from the following unit (where it had been placed by Damiani et al., 1998), due to the ubiquitous presence of the marker Campbelliella striata. The 'Campbelliella limestone' crops out in a limited area, close to Colle la Fossa. The short stratigraphic range of Campbelliella striata (upper Tithonianlower Berriasian), coupled with the absence of the Jurassic marker Clypeina jurassica in the analyzed samples, allow to ascribe this unit to the lower Berriasian. The thickness is about 100 m.
A long phase of subaerial exposure of the region is believed to have taken place during the Paleogene, producing the so called 'Paleogene hiatus' (Cipollari & Cosentino, 1995;Cosentino et al., 2010;Damiani et al., 1990Damiani et al., , 1991, although alternative models have recently been proposed (Brandano, 2017).
Carbonate production, characterized by heterozoan assemblages, was resumed starting in the early Miocene, producing a paraconformable contact with the Cretaceous substrate (Civitelli & Brandano, 2005).
The 'bryozoan limestone' (upper Burdigalianlower Tortonian p.p. -Civitelli & Brandano, 2005) is the last carbonate platform unit in the region and is a thick to fairly bedded white packstone with abundant bryozoans and bivalves. The rich micropaleontologic assemblages include abundant benthic forams, serpulids, barnacles, echinoid fragments with syntaxial calcite cement, and rare planktonic forams. Some levels are dominated by rodoliths. The upper portion is characterized by a peculiar lozenge-shaped fracturing pattern. The unit in the study area directly rests above the Cretaceous substrate and exceeds 100 m in thickness.
The 'brecce della Renga fm.' was subdivided in three lithofacies and six sublithofacies by Compagnoni et al. (1990Compagnoni et al. ( , 1991Compagnoni et al. ( , 2005, based on field geometries, rudite/ arenite/pelite ratio, and a diverse array of sedimentological features. A detailed description of each lithofacies and sedimentology of the 'brecce della Renga fm.' can be found in Fabbi and Rossi (2014).
Only lithofacies 2 and 3 of the 'brecce della Renga fm.' are found in the study area.
The lower Messinian lithofacies 3 of the 'brecce della Renga fm.' crops out along the NE slopes of the Simbruini Mts., and consists of stacks of dm-to mthick breccia beds and associated arenites, interbedded with the siliciclastic turbidites; the breccias often form disorganized intervals, up to some tens of meters thick (Figure 4) also bearing large boulders of the carbonate substrate. Graded and laminated turbiditic calcarenites are commonly organized in stacks up to some meters thick. The fossil content of the calcarenites include fragments of echinoids, balanids, bryozoans, molluscs, and red algae, along with benthic forams (Amphistegina sp., Elphidium sp., Planulina sp., Anomalinidae, Cibicididae, Rotaliidae, Textulariidae). The abundant siliciclastic fraction is essentially composed of quartz and micas. The maximum estimated thickness is a few hundred meters.
Post-emersion continental deposits are essentially Quaternary in age. Being the mapping project focused on the substrate, continental deposits have been separated only on the basis of sedimentological and positional features, and they are subdivided into three separate categories: i The deposits cropping out at valley bottoms (i.e. the Liri river alluvial sediments); the thickest soils (> 1 m) which commonly hide the bedrock in the inner valleys of the Simbruini Mts.; the volcaniclastic (essentially cineritic) deposits belonging to the Alban Hills or the Oricola volcanoes (Compagnoni et al., 2005;D'orefice et al., 2014) which occasionally are up to >5 meters thick ( Figure 6) in the inner valleys of the Simbruini Mts.); ii a wide landslide which affects the slope whose toe host the village of Petrella Liri, which is partly built on and around large fallen limestone boulders; iii slope debris, mainly composed of pebbles and boulders belonging to the carbonate succession and to the 'brecce della Renga fm.'.

Tectonics
At least three main tectonic phases in Miocene to recent times affected the study area: (i) A late Miocene extension, which originated the structural high whose dismantling produced the spectacular clastic deposits of the 'brecce della Renga fm.'; (ii) a latest Miocene-Pliocene compressional phase which gave origin to the Apennine chain; (iii) a Pleistocene post-orogenic extensional phase, which is still active in the Region, as confirmed by recent strong earthquakes. Pre-orogenic paleofaults are largely sealed by the 'brecce della Renga fm.', and are mapped with a different symbol. The best exposure of pre-orogenic extensional faults is to be found at Colle la Fossa, where the Cretaceous substrate is cut by SW-dipping faults (with total throw of a few hundred meters) sealed by the sublithofacies 2-a of the 'brecce della Renga fm.' (Figure 7 -Carminati et al., 2014). A similar situation, with the breccias onlapping a SWdipping normal fault paleoescarpment can be observed at Monna Rosa ( Figure 8 -Carminati et al., 2014;Fabbi & Rossi, 2014). For a detailed description and analysis of pre-orogenic faults and of the paleogeographic setting of the area the reader is referred to the papers of Carminati et al. (2014) and Fabbi and Rossi (2014).
The Simbruini Mts. are essentially a wide NE-dipping monocline. This monocline is cut to the E and NE by the Simbruini thrust front, and is fragmented to the W by a large system of regional SW-dipping extensional faults (Figure 1), largely exposed outside the study area (Carminati et al., 2014 and references therein).
The main compressional fault in the study area is the Simbruini thrust front, one of the most important thrusts in the region, which trends NW-SE along the Roveto valley, and becomes a blind structure south of Cappadocia. In the field, it is possible to observe at least two main splays and several minor lineaments. Good exposures of the westernmost lineament are at Verrecchie and along the Verrecchie-Cappadocia road. The best exposure of the easternmost lineament occurs near the Cappadocia cemetery (Figure 9), where the most external outcrop of the lithofacies 3 of the 'brecce della Renga fm.' is severely deformed, and overthrusts the siliciclastic sandstones of the 'complesso torbiditico altomiocenico laziale abruzzese'. Kinematic indicators measured along the whole structure, including minor fault planes, show a general apenninic vergence (N 60°E -Fabbi, 2016). A minor reverse fault trends NW-SE and crops out along the Fioio valley, at the southwestern boundary of the study area, causing the overthrusting of the 'Saifar limestone' above the 'Fioio dolostone'. In addition, several low angle reverse planes cut the verticalized breccia beds at Verrecchie and along the Capistrello-Piani della Renga-Filettino road, evidencing that the breccias should be already lithified when the region experienced compressional tectonics (Fabbi, 2013;Fabbi & Rossi, 2014).
As was mentioned above, the main extensional faults of this sector of the Apennines are located outside the mapped area, where, however, a number of  minor faults can be observed. It is worth noting that the pattern of faults affecting the substrate is more dense than the pattern seen in the 'brecce della Renga fm.', which suggests that the substrate records subsequent superimposed tectonic phases, including the one which generated the breccias (Carminati et al., 2014;Fabbi, 2013), while the breccias record only orogenic compression and subsequent Quaternary extension. A major regional tectonic lineament observed in the study area is the left-lateral transpressive fault system which borders the left side of the Roveto valley (Compagnoni et al., 2005;Montone & Salvini, 1993;Smeraglia, Aldega, Billi, Carminati, & Doglioni, 2016). According to Galadini (1999) and Roberts and Michetti (2004), this fault is now reactivated as an extensional fault and it is still active and seismogenic. Subvertical fault planes are well exposed along the valley (Figure 9), generally striking N 140°E.

Conclusions
A geological map on the 1:20,000 scale displays the geology of the eastern sector of the Simbruini Mts., in Central Apennines. Following a common picture for the Central Apennines, this sector of the Chain is characterized by a thick (> 3000 m) succession of Mesozoic and Miocene shallow-water carbonates, belonging to the Latium-Abruzzi carbonate platform Domain.
The carbonate sedimentation ended in the late Miocene, because of the involvement of the region in the Apennine chain building phase. The ongoing foreland flexure caused the gradual shift from carbonate to  siliciclastic sedimentation of the region, that became a complex foredeep basin, facing the Apennine orogenic system.
A result of the foreland flexure was a strong pre-orogenic extensional tectonic phase that affected the northern Simbruini Mts., producing a complex structural high within the basin. The dismantling of the flanks of such a high produced a thick lithoclastic unit (brecce della Renga fm.) which is widely exposed in the study area. This clastic unit is a unicum in the region and is partly lateral to the more typical foredeep succession of Central Apennines, represented here by turbiditic sandstones.
Since the Messinian the whole sedimentary succession was deformed and uplifted by the NE-verging Apennine building compressional phase. The final uplift of the chain caused the definitive halt of marine sedimentation in the region since the latest Messinian-Pliocene, and the sedimentation of continental deposits. The main orogenic structures in the map are the Simbruini thrust front, which becomes a blind structure south of the Cappadocia village, and the Roveto valley left transpressional system. The last tectonic event affecting the area is a SW-directed extensional phase, which is still active in the Central Apennines. Several normal faults cropping out in the area are interpreted as secondary lineaments related to such postorogenic extension.

Software
The map has been drawn using the Adobe Illustrator CS2 software, on scanned hand-drawn maps. The topographic base map is the Abruzzo Region D.B.T.R. on the 1:25,000 scale (sheets 367E, 376 and 377W), available online.

Acknowledgments
I would like to thank Massimo Santantonio for fruitful discussions and for the critical revision of the manuscript. Maurizio Chiocchini, Rita Pichezzi and Mariagrazia Rossi are warmly acknowledged for their willingness and their essential help in determining microfossils. The base maps have been retrieved from the Regione Abruzzo Cartographic Service Opendata (http://opendata.regione.abruzzo.it/). Finally I would thank all those who helped me during the fieldwork: Giulia Colasanti, Damiano Mangiacapra, Gianluca Massetti, Gaia Mascaro, Natalia Prosciutti, Benedetto Petricca, Antonello Simonetti and Eugenio Carminati.

Disclosure statement
No potential conflict of interest was reported by the authors.