Sandstone Petrology and Provenance in Fold Thrust Belt and Foreland Basin System

The sandstone composition of foreland basin has a wide range of provenance signatures, reflecting the interplay between flexed underplate region and abrupt growth of the accreted upper plate region. The combination of contrasting detrital signatures reflects these dual plate interactions; indeed, several cases figure out that the earliest history of older foreland basin infilling is marked by quartz-rich sandstones, with cratonal or continental-block provenance of the flexed underplate flanks. As upper plate margin grows over the underplate, the nascent fold-andthrust belt starts to be the main producer of grain particles, reflecting the space/ time dependent progressive unroofing of the subjacent orogenic source terranes. The latter geodynamic processes are mainly reflected in the nature of sandstone compositions that become more lithic fragment-rich and feldspar-rich as the foldthrust belt involves the progressive deepest portions of upper plate crustal terranes. In this context sandstone signatures reflect quartzolithic to quartzofeldspathic compositions.


Introduction
The evolutionary record of Earth's processes ascribed to sedimentary rocks has been pivotal in paleogeographical and paleotectonic reconstructions of source/basin systems. Compositional trends of clastic strata through space and time are used to frame the structural history of adjacent mountain belts and to monitor the key geodynamic changes during orogenic processes [1][2][3][4][5]. Clastic infilling of sedimentary basins, in orogenic systems, have been used as important indicators of tectonic activity and climatic changes. In the orogenic systems, clastic sedimentation may record the accretionary processes, the accommodation of the thrust units, and the flexural features of the foreland plate [5]. The chapter summarize the close relations between source to sink in orogenic-derived sandstones. Foreland basin stratigraphy is intimately connected with the growth of orogenic systems. The derived sandstones reflect the changing nature of fold-thrust belts and the flexural feautures of the underplate. The main goal of this contribution is to discuss the changing sandstone petrofacies during infill of foreland basin systems using a petrological approach.

Fold-thrust belts in orogenic setting and Foreland Basin systems
The development of an orogenic wedge during continental collision points out a thickening of the crust. The excess of mass in this thickened crust acts like a load on the underthrust plate; that drag it to a flexure downwards close to the load, showing out a foreland basin framework [6,7]. During plate convergence, the vertical load of the mountain belt migrates over the foreland plate, causing the migration of the associated foreland basin.
The foreland is the region between the front of a thrust belt and the adjacent craton [8][9][10][11]. Large volumes of clastic sediments derive from erosion of the thrust belt and subsequent deposition in the foreland basin. The foreland basin is generally defined as an elongate trough formed between a linear constructional orogenic belt and the stable craton, mainly in response to flexural subsidence caused by thrustsheet loading in the orogen (Figure 1).
Foreland basin stratigraphy records tectonic, eustatic, and climatic changes at convergent plate margins [11], where the formation of unconformities is the result of the interplay of temporal variations in the erosion and lateral progradation rates of the orogenic wedge, as well as tectonic and eustatic sea-level changes [6,[16][17][18].
In foreland settings, subsidence and uplift are profoundly affected by lithospheric flexure. Foreland basin subsidence is primarily controlled by downflexing of the lithosphere in response to thrust accommodation and loading [6,16,19]. Subsidence rate gradually decreases away from the thrust front, producing an asymmetrical depression. Flexure uplift (forebulge) occurs as an isostatic response to warping downward and forms the distal margin of the foreland basin. Cratonward the forebulge flexure, a broad shallow downwarp or intrashelf basin forms, named as back-bulge basin (Figure 1) [12,20].
The dimension and amount of flexural subsidence and uplift produced by the flexural features (i.e., foreland basin, forebulge, back-bulge basin) primarily depend on the geometry and density of the tectonic load, rheology of the lithosphere, density and volume of the sediment infill, and amount of thrust wedge and forebulge erosion [5,6,12,16,21]. The interrelationships between lithospheric flexure, single thrust accommodation within the accretionary wedge structure and flexural subsidence creates geometrical complex bodies within the foreland region. The foreland basin system may be divided into four depozones, the wedge-top, the foredeep, the forebulge, and the back-bulge depozones [12] (Figure 1). Boundary between depozones may shift laterally through time following the deformation propagation. The longitudinal dimension of the foreland basin system is roughly equal to the length of the adjacent fold-thrust belt [12].

Concepts of compositional signatures
The petrographic signatures during the different ages of the foreland infill show a clear sign of the diverse provenance sands, related to the different steps of the uplift stages and strictly connected to nearby rock portions exposed during the tectonic events. The discrimination of the principal source than the secondary one will point out the whole petrographic composition. This approach gives the base to the building of provenance and regional scale models from sandstone petrography.

Provenance models
Foreland regions are one of the typical setting in which huge volumes of clastic sediments are rapidly accumulated. This peculiar feature became a key element in provenance studies of such tectonic setting, because it consequently reveals several issues as the framework of the basin evolution complex history, sediment dispersal pathways, dating of major thrust events and the thrust-belt unroofing history [2,[22][23][24][25]. In this kind of setting, the uplift-erosion-transport-deposition system is genetically and intimately related to the deformation style in thin-skinned thrusted terranes. Transport of clastic sediment parallel to the tectonic shifting is the commonly assumed setting for the clastic-wedge/thrust association [23][24][25], named as «synthetic dispersal». However, opposite sediment dispersal pathways to the tectonic shifting is possible where hanging-wall beds dip toward the interior of the thrust belt, paleoslope, and therefore sediment dispersal, is opposite to tectonic transport. Such dispersal model, defined as «anthitetic dispersal» [25], reveals a kind of inverted stratigraphy, reflecting unroofing in the source, or mixed compositions. As result of these assumptions, it follows that sediment dispersal pathways in foreland basin systems are controlled by geometries within the thrust sheet system as frontal ramps, lateral ramps, and diverse hanging-wall beds dip. If distinct source-rock compositions are eroded sequentially, as in the case of predominantly vertical uplift of a stratigraphic section, «unroofing sequences» are commonly formed in the resultant anatomy of a clastic wedge [23,25,26].
This erosional inverted clast stratigraphy (unroofing history) can provide valuable information about the evolving sources and the identification of specific source areas [26][27][28][29][30]. In the case of thin-skinned thrusted terrains (where horizontal transport dominates), layered rocks with different lithologies are exposed to erosion as they pass over a ramp, providing a blended clastic dispersal pathways of the exposed rock types. The resulting clastics may show no unroofing sequences, but include the same blended clast composition for relatively great thickness. These blended clastics may indicate that the source rocks were formed by tectonic transport over a ramp [25]. In thin-skinned thrust belts, both «unroofing sequences» and «blended clastics» can result in combinations, due to the evolution of the entire orogenic system and foreland basin infill.

Large and regional-scale models based on sandstone Petrostratigraphy
Numerous studies have demonstrated that sand (stone) from foreland basins are characterized by high framework percentages of quartz and unstable sedimentary and metamorphic lithic fragments, and the mean composition is quartzolithic [1-3, 5, 30-32].
The foreland basin systems are a typical basin-setting in which multiple sources can be active at the same time, and the derivative sandstones may show mixed petrofacies [4,5,31,33] (Figure 2). Schwab [31], in a general statement of QmFLt (Qm = monocrystalline quartz, F = feldspars, Lt = aphanitic lithic fragments) diagram illustrate the concept of mixing detritus from different provenance types that produce detrital modes reflecting mixed provenance [2]. Typical foreland-basin sand suites were derived from uplifted fold-thrust belts exposing sedimentary and metasedimentary strata. The mixed provenance relations are also typical of some foreland basin systems and remnant ocean basins (i.e. southern Apennines foreland, Indus and Bengal fans of the Himalayan belt). During early stage of foreland infill, sand may derive from cratonal areas, generating quartzose sand. Subsequent petrofacies is quartzolithic, and during final foreland infill (foreland uplift), petrofacies may be mixed and quartzofeldspathic. Modified after [4,15].
foreland-basin sandstone petrofacies, testifies the complex pattern of provenance relationships during the foreland basin evolution. Quartzose sand is typical during the early stage of foreland infill, when the thrust-belt has low elevation and consequently supplies low amounts of detritus, while cratonal region is flexing and provides more amounts [33,34]. The subsequent petrofacies is typical quartzolithic, when the thrust-belt is growing and show up its roots. Local nearby provenances from magmatic arcs, uplifted subduction complexes or uplifted carbonate rocks of the forebulge represent just small amounts of the clastic record within foreland basin system. Only if the thrust belt shows severe uplift rates, in a way to expose the crustal basement, petrofacies can evolve to quartzofeldspathic sand during the late stage of "Foreland Uplift" [3][4][5][35][36][37].

Provenance and setting: Sandstone detrital modes and orogenic provenance
Foreland basins adjacent to orogenic wedges experience drastic changes in provenance during their sedimentary history due to the high stressed event. In general models, arkosic petrofacies of rift phases lie at the base of the foreland successions, followed up by quartzose petrofacies of the passive margin or cratonal regions. These pre-orogenic strata are then succeeded by quartzolithic petrofacies derived from upper crust thrusted units of the nascent orogenic system. Detrital modes in foreland setting are the combination of two main key sources located on the thrusted and uplifted upper plate, and on the underplate. These issues are crucial in the definition of key tectonostratigraphic sources that grow up from the deformational pattern of the plate convergence, closely revealed in the clastic stratigraphy of the resulting foreland basin system. Sandstone petrology is a key tool for unrevealing the close relation between source to sink and the spatial and temporal significance of the whole detrital budget.

Upper plate sources
Thin-skinned thrust terranes. -Such terranes are mainly made up of metasedimentary bedrocks and sedimentary successions directly covering their basement rock suites; other thrusted units are composed by ophiolitiferous sources of ancient consuming oceanic basins, including oceanic crust rocks of both metavolcanic and subvolcanic fragments and their oceanic sedimentary cover of chert-rich, pelagic shale and limeclasts (Figure 3).
Foreland Uplift terranes. -Mid-crustal deformed and thrusted terranes, including mid-high-grade metamorphic rocks (micaschist, gneiss, granulite) and plutonic suites (mainly tonalite-to-granite) are involved in the plate convergence and rapidly exhumed and uplifted. These thick-skinned thrust terranes are high producer of sand, indeed, phaneritic crustal terranes are able to generate huge volumes of feldspar-rich clastic material, and they are responsible of detrital mode shifting toward more quartzofeldspathic sandstone suites within the orogenic provenance field. Many examples along the stratigraphic record testify the changing nature of foreland sandstone petrofacies from quartzolithic to quartzofeldspathic sand suites [5,29,30] (Figure 4).
Remnant Volcanic arc. -Persistent active volcanic sources, as remnants of precollisional history, may contribute to the sedimentary budget of the foreland basin system when volcanic activity continues during the early stages of foreland evolution. Signatures from active volcanic sources may be diluted with other detrital budget or they can represent distinctive volcaniclastic layers interbedded within the foreland clastic wedge ( Figure 5).

Underplate sources
Thrusted underplate margin. -Portions of the underplate continental margin may be involved in flexure and in underthrust, and then assembled within the orogenic wedge. Usually they include sedimentary cover and upper crust stratigraphy of the underplate. Shallow-water to deep-water strata and upper crust low-grade Forebulge. -It is the region of potential flexure uplift along the craton side of the foredeep. Because of forebulge is a positive and potentially migratory feature, which may be eroded, its potential of preservation is low. A signal of the presence of ancient forebulge may be the erosional unconformity surface. The forebulge is generally considered a zone of nondeposition or erosion, or a condensed succession, and the resulting unconformity may be used to mark its location through time. In subaerial foreland basins the forebulge is a region of erosion, with streams draining both toward and away from the orogenic belt. In submarine foreland basin systems, local carbonate patch may be developed; extensive forebulge carbonate strata can connect the foredeep with the back-bulge depozone [13]. Forebulge is a producer of sediments for the foredeep in terms of delivering detrital grains and huge gravity flow deposits. The nature of the detrital budget is in response of the underplate stratigraphic record (Figure 6).
Internal stable foreland. -Craton interior region of the underplate widely contributes to the initial stages of foreland generation, and usually discharge huge volumes of supermature quartzose detritus into the foreland infill. Local limeclasts and intrabasinal carbonates are an adding source (Figure 7).

Key sandstone petrofacies filling foreland basin
Key sandstone petrofacies reflects the changing nature of the orogenic evolution and related filling of foreland basin. Quartzose, quartzolithic, quartzofeldspathic, and hybrid sandstones are the main signatures in foreland setting (Figure 8). Local calclithite and volcaniclastic sandstones may also occur in this tectonic setting. Optical analysis of modal point count in sandstone is hardly suggested for the best fitting of the whole composition trying to recognize the temporal and spatial significance of the identified petrographic classes ( Table 1).

Quartzose petrofacies
Quartz-rich sandstones reflect provenance from stable craton and lowlands reflecting abundance of rounded quartz grains, mainly monocrystalline. The stratigraphic record testifies occurrence of quartzose petrofacies as the main signatures

Quartzolithic petrofacies
Elongated Q-L plotted quartzolithic suites is the main composition of foreland sandstones reflects the growing orogen of thin-skinned thrust belt of accreted plate. Large occurrence of metasedimentary and sedimentary lithic fragments, such as  [3,35], as such as many other sandstone examples of Paleozoic through Pleistocene orogens [5,11,15]. On both ancient and recent foreland sand (stone), the main detrital component is the abundance of quartz and aphanitic lithic fragments. The latter are mainly represented by (i) low-to-medium grade  [38], and (b) later refinement by Dickinson [1].

Quartzofeldspathic petrofacies
Plotted in the intermediate portions of the QFL diagram, sandstones having abundance of quartz and feldspar and minor lithic fragments are quartzofeldspathic sandstone suites. They reflect the uplift foreland stage of late orogenic phases when mid-crustal rocks (dominantly high-grade metamorphic and plutonic suites) are involved in thrusting, exhumation and uplift. Key lithostratigraphic units in the geological record have quartzofeldspathic sandstone suites as Old Red Sandstone of the Caledonian orogen [36] or New Red Sandstone of the Variscan orogen (e.g. Val Gardena Fm.) [44], as such as late petrofacies units of many other orogens [5,35].

Interbedded volcaniclastic petrofacies
Coeval volcaniclastic sandstones may accommodate in foreland region due to persistent volcanism, related to the subduction during closure of remnant ocean basins and early foreland stage. Dominantly arc-derived volcaniclastic sandstones occur in foreland setting as important detrital contribution in quartzolithic suites and as interbedded volcaniclastic suites, related to the coeval volcanism with sedimentation. Volcanic debris is represented mainly as abundance of vitric fragments (ash shards) in ash turbidites or as occurrence of typical textural attributes in volcanic lithic fragments [3]. The most typical volcanic textures in foreland strata are vitric, felsitic granular, felsitic seriate and microlithic reflecting typical silicic volcanism from trachite-andesite to rhyodacite and rhyolite.

Hybrid arenite petrofacies
Hybrid arenites are defined by Zuffa [39][40][41]45] as the mixing of extrabasinal detrital grains, both siliciclastics and carbonate, and intrabasinal detrital grains, both noncarbonate and carbonate. In many submarine foreland settings, the mixing of intrabasinal and extrabasinal detrital grains reflects hybrid arenite suites. Mostly is the combination of siliciclastic particles together with the main allochemical grains [46] or intrabasinal carbonate grains. These signatures are in response of remobilized intrabasinal grains during huge arrival of subaerial clastics, temporally stored in shallow water. Deep-water turbidite systems in foreland basins include large occurrence of hybrid arenites suggesting the active rules of shallow-marine environment. Also, non-carbonate -rich (mainly glaucony and phosphatic) hybrid arenite signatures testify maximum flooding during sea-level changes [47].

Calclithite petrofacies
Limeclasts [38] and extrabasinal carbonate grains [39][40][41] signatures are typical in foreland settings [48,49]. Ancient carbonate grains derive from eroded older carbonate strata and reflect, in foreland setting, diverse but interfingered dispersal pathways drained from the fold-thrust belt, intrabasinal structural highs, forebulge and the stable foreland. Distinguishing the nature and location of older carbonate detritus is crucial for detailed paleogeographic reconstructions. Older carbonate source rocks may occur as both (i) subaerial exposed carbonate strata interbedded within the fold-thrust belt, and as (ii) intrabasinal structural highs within wedgetop and foredeep depozones, that can result as base-of-slope carbonate breccia and related turbidite sandstones. Within the circum-Mediterranean region, ancient foreland sandstone strata reflect important contributions from older carbonate detritus in the Pyrenees [50], Betic Cordillera [42,51], Iberian Range [52,53], Alps and Apennines [5,48,49,54], due to large abundance of Mesozoic-to-Cenozoic carbonate platform/to basinal successions related to the Neo-Tethyan oceanic rifting.

Spatial and temporal significance of sand grains
More refined compositional signatures of sand (stone) in foreland setting include temporal (coeval vs. noncoeval) and spatial (extrabasinal vs. intrabasinal) decoding of clastic particles [40,41]. The large spectrum of detrital grains in sand (stone) has high value in inferring the spatial/temporal constraints. Carbonate and volcanic detrital grains are particularly sensitive of these discriminant subdivisions if the ultimate goal is the correct palaeogeographic reconstructions. For instance, carbonate grains can be spatially generated in extrabasinal and intrabasinal environments, and can be noncoeval (paleo) or coeval (neo) with sedimentation: (i) noncoeval extrabasinal carbonate grains are eroded and grains generate carbonate lithic fragments (dolostone and limestone); (ii) coeval extrabasinal carbonate grains are fragments of nowely formed carbonate concrections in soils (calcrete, caliche) or travertine fragments; (iii) coeval intrabasinal carbonate grains are the typical allochemical particles [46] including ooids, bioclasts, intraclasts and peloid; (iv) noncoeval intrabasinal carbonate grains include older carbonate strata in structuralrelated intrabasinal highs that can deliver large blocks and sand at the base of slope.
Volcanic particles represent the most intricate task in optical analysis for the discrimination between grains eroded from ancient volcanic rocks (paleovolcanic or noncoeval grains) and grains generated by intrabasinal or extrabasinal active volcanism during sedimentation (neovolcanic, coeval grains) [40,41,55]. Apart the contributions from older eroded volcanic rocks that refine the general puzzle of source areas, the coeval volcaniclastic contribution represent a well-defined marker in the sedimentary record. Volcanic particles can reveal important constraints on deciphering spatial (extrabasinal vs. intrabasinal) and temporal relationships of neovolcanic events (pre-, syn-, inter-and post-eruptive periods).

Summary
The whole chapter tries to summarize the typical fold thrust belt and foreland basin system settings from the petrological point of view of the key sandstone suites. This kind of reasoning permits to point out all the evolutionary events of these tectonic frameworks. After explaining the teorethical key concepts of the tectonic structures involved and the main provenance models, the discussion points rawly to the key sandstone petrofacies that commonly represent the infilling of the basin settings. The early stage of the foreland usually reflects quatzose sandstone suites derived from cratonal area. The huge volumes of the foreland infill has quartzolithic sandstone suits, derived from the growing orogenic belt. Finally quatzofeldspathic sandstone compositions are mainly derived from uplifted thrust belt. Local volcaniclastic and hybrid arenites can also occur during the foreland history.

Conclusions
In foreland settings, subsidence and uplift are profoundly affected by lithospheric flexure. Foreland basin subsidence is primarily controlled by downflexing of the lithosphere in response to thrust accommodation and loading. The interrelationships between lithospheric flexure, single thrust accommodation within the accretionary wedge and flexural subsidence experiences geometrically complexes entities within the foreland region [4,5]. Sandstone signatures of foreland-related basins reflect the close changing nature of detrital composition during the growth of the orogenic belt and the flexure of the underplate. Quartzolithic sand (stone) suites are the main composition of foreland sandstones during syntectonic evolution of foreland infill. However, quartzose suites occur during the early stage of foreland infill, as such as quartzofeldspathic sandstones during the later foreland uplift. Local contributions of volcanolithic sandstones from magmatic arc may occur as remnant arc activity continues during initial orogenic processes. Finally, carbonate-rich strata derived from both erosion of ancient carbonate successions and coeval carbonate/noncarbonate detritus are typical in many marine foreland infill.
In conclusions, close relations between regional sandstone petrofacies and major unconformities may contribute to the more precise paleogeographic and paleotectonic reconstructions. and Earth Sciences at the University of Calabria. This research was funded by MIUR