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Geology of the Northern Simbruini Mts. (Abruzzo – Italy)

ABSTRACT This paper presents the results of a geological mapping project across the northern portion of the Simbruini Mts. (Latium-Abruzzi Domain – Central Apennines), where a thick Cretaceous and Miocene carbonate succession, followed by a thick upper Miocene terrigenous foredeep succession, is exposed. The terrigenous succession also includes a peculiar lithoclastic unit (‘brecce della Renga fm.’), whose sedimentation is linked to pre-orogenic (Tortonian-Messinian) extensional tectonics. The study area experienced late Messinian-early Pliocene compression, which is the Apennine chain building phase, followed by Quaternary post-orogenic extension, related to the opening of the Tyrrhenian basin. A geological map, at 1:20,000 scale, illustrates the main stratigraphic and structural features of the area.

The mapping project was part of a research project aimed at the reconstruction of the Miocene paleogeography of the study area, and to the sedimentological study of the 'brecce della Renga fm.'. For this reason a new geological map was produced, although the study area has already been mapped (sheet #367 'Tagliacozzo' of the geological map of Italy at 1:50,000). There are, thus, along with many obvious similarities, some differences between the map presented here and the official geological map, mainly due to the more detailed field surveying scale (1:10,000 vs. 1:25,000), which allowed better outcrop representation (including those of limited extent) and to a new interpretation of some tectonic and stratigraphic features. The main difference is the separate representation of each sublithofacies of the 'brecce della Renga fm.' Making is easier to identify the present-day distribution of these deposits, which are controlled by the Miocene paleogeography (see below). In addition, some faults have been reinterpreted as paleofaults, buried by the 'brecce della Renga fm.' instead of cutting them, and the pattern of tectonic lineaments is now better constrained with new and more detailed field data.

Methods
The map is the result of a geological survey originally performed at 1:10,000 scale, using an enlarged 1:25,000 IGM topographic map (Series 25, year of publication 1994. Sections: 367 II 'Tagliacozzo'; 367 III 'Arsoli'; 367 IV 'Carsoli'). A lithostratigraphic criterion has been used for this study, accompanied by biostratigraphic analyses (in thin section), and integrated with sedimentological analysis for the terrigenous deposits (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 as those described in Compagnoni et al. (2005); minor differences concern the stratigraphy of the terrigenous units, which have recently been reviewed for the scope of the CARG (Geological mapping of Italy) project.

Stratigraphy and geological setting
The stratigraphy of the study area ( Figure 2) is regionally known as the Latium-Abruzzi succession, and reflects the evolution of the Latium-Abruzzi carbonate platform, which was the site of shallow-water deposition since the Late Triassic to the middle Miocene (Accordi & Carbone, 1988;Chiocchini et al., 2008;Damiani, 1990;Damiani, Catenacci, Molinari, Panseri, & Tilia, 1998;D'Argenio, 1974;Parotto & Praturlon, 1975. In the late Miocene the study area became involved in the Apennine chain orogenic phase, which caused the definitive halt of shallow-water carbonate sedimentation and the development of a thick syn-orogenic (foredeep) terrigenous succession, essentially Tortonian-early Messinian in age (Bigi et al., 2003;Carminati, Fabbi, & Santantonio, 2014;Compagnoni et al., 2005 and references therein). Due to the orogenic uplift, the majority of the area emerged during the Pliocene, and the outcropping continental deposits are essentially Quaternary in age.
In this section the main features of the stratigraphic units cropping out in the study area, and their significance in the general geodynamic setting of the region, are briefly discussed.
The youngest Cretaceous carbonate unit in the study area is the uppermost Campanianlower Maastrichtian 'orbitoid limestone' (Figure 3), a thin (zero to few tens of meters) recrystallized packstone, organized in dm-to 1 m-thick beds and characterized by oligotypic faunas, with abundant Orbitoides media and Orbitoides spp.
The lithofacies 1 is a pelite-arenite-rudite association (Figure 7(a-d)), with pelites often dominating, and is widely exposed in the northern sector of the study area. It is subdivided into four sublithofacies (1-a, 1-b, 1-c, 1-d) mainly based on varying pelite/ rudite ratio (Fabbi & Rossi, 2014). On the map the sublithofacies 1-b and 1-c have been grouped based on the transitional nature of the boundary, which is poorly exposed in the field. The lithoclasts in this lithofacies are both Miocene and subordinately Cretaceous limestone, and also include large olistoliths of 'bryozoan limestone' (Figure 7(a)). The arenites and the breccia matrix are composed of fragments of bivalves, echinoids, balanids, bryozoans and red algae, along with Ditrupa sp., benthic forams and rare planktonic forams; the main siliciclastic components are quartz and micas .
The lithofacies 2 of the 'brecce della Renga fm.' (Figure 7(e-i)) is the most widely exposed and is subdivided into a massive sublithofacies (2-a) (Figure 7 (e-g)) and a well bedded sublithofacies (2-b) ( Figure  7(h-i)). The sublithofacies 2-a outcrops extensively in the study area, with a total thickness of more than 300 meters (Fabbi & Rossi, 2014) and is made of clast-supported carbonate breccias. The clasts are markedly heterometric, ranging from sand grains to boulders (up to tens of meters across). This lithofacies rests unconformably on the Lower Cretaceous/Miocene substrate (Fabbi, 2013;Fabbi & Rossi, 2014). As the breccias were sedimented through low-efficiency processes (rockfall, rock-avalanche, grainflow) their composition is strongly influenced by the local substrate: Cretaceous clasts are dominant wherever the breccias are surrounded and overlie a Cretaceous substrate (i.e. along Miocene escarpments the Cretaceous rocks were exhumed), while Miocene clasts are almost exclusive wherever breccias lie on the Miocene substrate (i.e. Miocene faults/escarpments were shallower and the Cretaceous was not exhumed) (Fabbi & Rossi, 2014). Along with lithoclasts, Miocene granules include coeval intrabasinal isolated echinoids, bivalves, benthic forams and bryozoans (Figure 7). This unit commonly lacks any stratal organization, so even bed attitude is difficult to detect. A peculiar character of this sublithofacies is the presence of yellow pelite intercalations (Compagnoni et al., 1990(Compagnoni et al., , 2005Devoto, 1967Devoto, , 1970Fabbi & Rossi, 2014;Parotto, 1969), which provide the essential biostratigraphical elements to determine the age of the rudites (early Tortonian-early Messinian -Fabbi & Rossi, 2014). In the inner (western) portions of the study area the sublithofacies 2-b of the 'brecce della Renga fm.' is typically well bedded, with a fining upwards trend, and with an upward increase of siliciclastic components (Compagnoni et al., 1990). Large ostreids, pectinids, balanids, echinoids and bryozoans are common intrabasinal components of the breccias and rounded chert clasts also occur. The matrix of the breccia is composed of skeletal grains such as fragmentary balanids, bryozoans, bivalves, echinoids, red algae, rare benthic forams and abundant siliciclastic grains (mainly quartz).
The third lithofacies of the 'brecce della Renga fm.' (Fabbi & Rossi, 2014) does not crop out in the study area.

Continental deposits
The final uplift and emersion of this sector of the Central Apennines occurred through the Messinian and late Pliocene, followed by a SW-directed extension of its inner portions, linked with the opening of the Tyrrhenian basin (Carminati & Doglioni, 2012;Doglioni et al., 1999;Gueguen, Doglioni, & Fernandez, 1998).
Pleistocene to Holocene continental deposits are distinguished in three separate units:  occasionally can be some meters-thick in the inner valleys of the Simbruini Mts.); (ii) a wide complex landslide which affects the northern slopes of Mt. Fontecellese, developed within shaly and marly lithologies; (iii) slope debris, mainly composed of pebbles and boulders belonging to the carbonate succession and to the 'brecce della Renga fm.'.

Tectonics
This section contains a brief overview of the structural setting of the study area. The area was affected by at least three main tectonic phases in Miocene to recent times: (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 is the origin of the Apennine chain and (iii) a Pleistocene post-orogenic extensional phase, which is still active in the western sectors of the Apennines. Strike-slip tectonics is commonly documented in the area (Compagnoni et al., 2005;Montone & Salvini, 1993); faults showing an important oblique slip are related both to the orogenic and the post-orogenic tectonic phases (Compagnoni et al., 2005;Montone & Salvini, 1993;Roberts & Michetti, 2004). Paleofaults interpreted as pre-orogenic are essentially sealed by the 'brecce della Renga fm.', and are mapped with a different symbol.
For a description and analysis of pre-orogenic faults and paleogeographic setting of the area see Carminati et al. (2014) and Fabbi and Rossi (2014).
The Simbruini Mts. can be described as a wide monocline, with beds roughly dipping toward the NE; bed attitude abrubtly steepens, up to vertical, close to the thrust front of the structure. The monocline is cut to the E and NE by the Simbruini thrust front and dismembered to the west by a large system of major (regional) SW-dipping extensional faults (Figure 1), whose plains crop out outside the study area (Carminati et al., 2014 and references therein).
Only secondary extensional faults ascribable to the latter tectonic phase have been identified in the study area.
The main structural element in the study area is the Simbruini thrust front, one of the most important thrusts in the region, which trends roughly W-E in its northern portion and NW-SE along the Turano river valley, continuing southwards outside the map. In the field, it exists as a wide tectonized belt where it is possible to observe at least two main thrusts ( Figure 8) and several minor lineaments (the latter having average throws of some tens of meters). The thrusts are often not clearly observable in the field, but they can be identified using alignments of cataclasites; in other cases their existence has been inferred in spite of poor exposure based on a 'geological necessity' (i.e. covered formation boundaries which are arguably not of stratigraphic nature). Kinematic indicators measured along the whole structure, including minor fault planes, show a general apenninic vergence (N 60°E -Fabbi, 2013).
Although the main extensional faults of the Simbruini ridge are located outside the mapped area, several minor faults are present in the mapped zone, most of them characterized by an important oblique component (transtensional faults -Fabbi, 2013). The main valleys in the study area are bordered by normal faults with throws generally ranging from some tens to few hundreds meters. Remarkably, in the westernmost portion of the map the succession is dissected by several small faults, making it difficult to determine the kinematics and the deformation history of this sector. The intense tectonization and fragmentation is possibly due to the superimposition of subsequent tectonic phases (Carminati et al., 2014;Fabbi, 2013). A major regional tectonic lineament which crops out in the study area is the left-lateral transpressive fault system bordering the Carseolani Mts. (Figure 8) which, according to Roberts and Michetti (2004), is still active, although reactivated as an extensional fault. This lineament has been described by Montone and Salvini (1993) and Compagnoni et al. (2005).

Conclusions
A geological map on the 1:20,000 scale is presented here, displaying the geology of a complex sector of the Apennine chain, where a thick Cretaceous and Miocene shallow-water carbonate succession crops out extensively, along with upper Miocene terrigenous units deposited in the foredeep basin produced by the advancing Apennine orogenic system.
A pre-orogenic extensional phase caused the formation of a prominent structural high in a region, which roughly corresponds to the present-day northeastern Simbruini Mts. The syn-sedimentary normal faults have been exhumed and can be mapped in the field. The syn-tectonic dismantling of the margins of the structural high produced a thick lithoclastic succession which represents a unicum in the central Apennines and is made of calcareous breccias and associated pelite/arenite intervals. This clastic unit is partly lateral to the typical foredeep succession of the Central Apennines, represented by hemipelagites and turbiditic sandstones. The whole sedimentary succession was deformed and eventually exposed subaerially by the NE-verging Apennine building compressional phase, and subsequently dissected by SW-directed extension. The main structures ascribable to the latter phase crop out outside the study area. The main orogenic structure on the map is the Simbruini thrust front, while several normal and transtensional faults are interpreted as secondary lineaments related to the post-orogenic extension.

Software
The map was produced using Adobe Illustrator CS2 from scanned hand-drawn maps. The topographic basemap is the Abruzzo Region CTR at 1:25,000 scale, available online.