Foreland Ramp Research Articles

Large-scale mass-transport deposits recording the collapse of an evaporitic platform during the Messinian salinity crisis (Caltanissetta basin, Sicily)

The Messinian primary gypsum sequence, formed during the first stage of the salinity crisis in the Caltanissetta basin (Sicily), is characterized by a high degree of disruption and deformation. The primary gypsum occurs as huge (up to 3 km large and up to 250 m high) tilted blocks, detached from their stratigraphic base, floating within a matrix of folded shales and clastic gypsum. This association represents a giant chaotic unit lying unconformably on the Gela Nappe, the youngest structural unit of the Maghrebian fold-and-thrust belt. The origin of the deformations affecting the Primary Lower Gypsum unit is still debated; in the past they have been related to intra-Messinian tectonics or to the collapse of primary gypsum platforms due dissolution of interbedded to halite. Our study on the distribution, size and bedding of the gypsum blocks reveals a pattern similar to that of modern giant submarine mass-transport deposits, which may have been triggered by a combination of active tectonics and sea-level change, and favored by the strong mechanical contrasts of the involved units. The chaotic unit shows a downslope evolution from the south, characterized by elongated blocks with their main axes parallel to the inferred slide headwall (NW-SE oriented), to intermediate and distal areas, where the blocks become progressively smaller in size with their main axes parallel to the flow (SW-NE). We identify the source area as either a shallow evaporitic basin above the Gela Nappe or on the Pelagian foreland ramp. A general implication for the Messinian salinity crisis is that no primary gypsum was deposited in deep water basins during the first stage; instead, during the second stage, large gypsum blocks collapsed from the basin margin and were mass-transported into a deeper water setting, where halite was depositing.

Timing of Contractional Tectonics in the Miocene Foreland Basin System of the Umbria Pre-Apennines (Italy): An Updated Overview

A large dataset of lithostratigraphic and biostratigraphic data, concerning the Early-Late Miocene turbidite succession of the Umbria pre-Apennines, is presented and analyzed. The data come from the study of 24 sections that are representative of all the main tectonic units cropping out between the front of the Tuscan allochthon and the Umbria-Marche calcareous chain. The sections have been dated using quantitative calcareous nannofossil biostratigraphy and, wherever possible, they were correlated through key-beds recognition. Such a multidisciplinary approach allowed us to reconstruct the evolution of the Umbria foredeep over time and to unveil the chronology of compressive deformations by defining: (i) the onset of the foredeep stage in each structural unit, (ii) the age of depocenter-shifting from a unit to the adjacent one, (iii) the progressive deactivation of the western sector of the foredeep due to the emplacement of allochthon units, and (iv) the internal subdivisions of the basin due to the presence of foreland ramp faults or thrust-related structures. A further original outcome of our study is having brought to light the Late Burdigalian “out-of-sequence” reactivation of the Tuscan allochthon which bounded westward the foredeep, and the subsequent protracted period of tectonic stasis that preceded the deformations of the Umbrian parautochthon.

Stratigraphic and structural reconstruction of an offshore sector of the Hyblean Foreland ramp (southern Italy)

Through the seismo-stratigraphic analysis of new high- resolution seismic data acquired along the southwestern offshore of the Hyblean Plateau, this work aims to improve the knowledge about the stratigraphic and structural setting in the marine area connecting the Hyblean Plateau Foreland to the Gela-Catania Foredeep.Two main goals have motivated the acquisition of new seismic profiles along the southern coastline of Sicily, in the Marina di Ragusa offshore:(i) to obtain a better comprehension of the Cenozoic stratigraphic and structural setting of the area, with a particular attention to the characterization of the Pliocene-Holocene sedimentary deposits and their areal distribution;(ii) to identify, in the offshore area, the possible prolongation of the main structural lineaments outcropping in the hinterland (Scicli-Ragusa-Irminio Line), and verify the presence of structures, responsible for the tectonic activity affecting the area.Our model shows that an extensional fault system, characterized by a main NE-SW orientation, affects the highly deformed Oligo-Miocene, or older, substratum, originating structural highs and down-faulted sectors. These faults are inferred to record the early history of the Scicli Line and of the polyphase kinematic evolution of the N50° oriented regional fault systems.The Gessoso-Solfifera deposits (Messinian, Late Miocene) have been recognized in several sectors of the study area, showing a very peculiar seismic facies, and occupying deep erosional channels probably resting within down-faulted sectors. This feature is in good agreement with isolated onshore areas of the Hyblean Plateau (Licodia Eubea and south Vittoria villages) where the Gessoso-Solfifera deposits are associated to normal faults, thus documenting that Messinian evaporites are not restricted to compressive tectonic setting, such as the marginal sub-basins and the thrust top mini-basins of the Appennine-Maghrebian belt.A peculiar seismic facies has been associated to the Trubi Fm., unconformably lying upon the older succession.The undeformed seismic units, onlapping the older substratum, has been associated to the post-Trubi-Holocene deposits, arranged into seven seismic units that reflect the youngest depositional evolution of the nearby Catania-Gela Foredeep. These deposits, generally represented by plane-parallel seismic facies, are in turn separated by an unconformity highlighted by an onlap termination of the yougermost terms upon the older. In some cases they are interrupted by gas rising from the deeper succession that reaches, at times, also the seafloor.

Mass Transport Deposits and geo-hazard assessment in the Bradano Foredeep (Southern Apennines, Ionian Sea)

Seafloor bathymetry, combined with multi-scale seismic reflection profiles, were used to describe the morphostructural setting of the Bradano Foredeep (Gulf of Taranto, northern Ionian Sea), where a submerged portion of the Southern Apennines is facing the Apulia Platform in the Calabrian Arc convergent margin. In this complex area, marine geophysical data highlight the presence of two mega-slide deposits at the shelf-slope transition, which are most likely the largest ever described in the region. These slid masses, named the Bradano Basento MegaSlide (BBMS) and Bradano Basento MegaSlide 1 (BBMS1), form a Mass Transport Deposit Complex (MTDC) affecting and eroding the topmost portion of the outer Apennines deformation front and the Apulian Foreland Ramp. It was emplaced in late Pleistocene times inside the narrow (about 10 km wide) Bradano Foredeep basin, a Plio-Pleistocene submarine trough developing at the chain front. The youngest of these deposits (BBMS1) likely predates the Last Glacial Maximum. Location of the MTDC within the subduction complex suggests that active tectonics and seismic shaking might represent the main triggering mechanisms for gravitative instability in this area, although gravitational tectonics and sediment creeping mechanisms cannot be excluded. On the other hand, size and distribution of the MTDC in the sedimentary record of the Bradano Foredeep suggest the need of re-evaluating the potential for large earthquakes/tsunamis, and more in general geological hazard related to submarine sliding masses, along the coast of this highly populated area.

Turbidites facies response to the morphological confinement of a foredeep (Cervarola Sandstones Formation, Miocene, northern Apennines, Italy)

AbstractThe Cervarola Sandstones Formation, Aquitanian–Burdigalian in age, was deposited in an elongate, north‐west stretched foredeep basin formed in front of the growing northern Apennines orogenic wedge. As other Apennine foredeep deposits, such as the Marnoso‐arenacea Formation, the stratigraphic succession of the Cervarola Sandstones Formation records the progressive closure of the basin due to the propagation of thrust fronts towards the north‐east, i.e. towards the outer and shallower foreland ramp. This process produces a complex foredeep that is characterized by syn‐sedimentary structural highs and depocentres that strongly influence lateral and vertical turbidite facies distribution. This work describes and discusses this influence, providing a high‐resolution physical stratigraphy with ‘bed by bed’ correlations of an interval ca 1000 m thick, parallel and perpendicular to the palaeocurrents and to the main structural alignments, on an area of ca 30 km that covers the proximal portion of the Cervarola basin in the northern Apennines. The main aim is to show, for the first time ever, a detailed facies analysis of the Cervarola Sandstones Formation, based on a series of bed types that have proven fundamental to understand the morphology of the basin. The knowledge of the vertical and lateral distribution of these bed types, such as contained‐reflected and slurry (i.e. hybrid) beds, together with other important sedimentary structures, i.e. cross‐bedded bypass facies and delamination structures, is the basis for better understanding of facies processes, as well as for proposing an evolutionary model of the foredeep in relation to the syn‐sedimentary growth of the main tectonic structures. This makes the Cervarola Sandstones, like the Marnoso‐arenacea Formation, a typical example of foredeep evolution.

Palaeovalleys in foreland ramp settings: what happens as accommodation decreases down dip?

AbstractRecent advances in our understanding of palaeovalleys are largely guided by examples from passive margins, in which accommodation increases down depositional dip. This study tests these models against a dataset from the Pennsylvanian Breathitt Group of the central Appalachian foreland basin, USA. This fluvio‐deltaic succession contains extensive erosionally based fluvio‐estuarine sand bodies that can be tracked over 80 km down depositional dip from a proximal zone of high accommodation close to the orogenic margin to a distal, lower accommodation zone close to the cratonic margin of the basin. The sand bodies are up to 25 m thick, multi‐storey and characterized in their lower parts by strongly amalgamated storeys containing sandy fluvial to estuarine bar accretion elements, and in their middle to upper parts by more fully preserved storeys up to 10 m thick and laterally extensive over 100s of metres. The upper storeys include abandonment channel‐fills of heterolithic marine or marginal marine deposits or muddy to sandy point‐bar elements. Three major regional‐scale architectures include: (i) Tabular sand bodies that everywhere incise open marine prodelta and mouth bar facies and are interpreted as palaeovalleys formed during falling stage and lowstand systems tracts, when eustatic sea‐level fall outpaced tectonic subsidence across the entire study area. (ii) Sand bodies that incise genetically related floodplain lake and/or bay‐fill minor mouth bar deposits up depositional dip and open marine prodelta and mouth bar facies down dip. These stacked distributary channel deposits map down dip into palaeovalleys and formed when up dip subsidence rate resulted in positive, but reduced rate of accommodation creation, while lower tectonic subsidence rate down‐dip resulted in incision. (iii) Sand bodies that incise genetically related floodplain, lake and/or bay‐fill minor mouth bars up dip and pass down‐dip into genetically related unconfined floodplain, prodelta and mouth bar deposits. These sand bodies represent stacked distributary channel fills and channel amalgamation was the product of high rates of lateral migration, typical of the behaviour of channels above their backwater reach. Case (2) sand bodies demonstrate that in rapidly subsiding foreland basins, cross‐shelf palaeovalleys may form down depositional dip from aggradational, distributive fluival strata. Additionally, the genetic relationship between stacked distributary channels and palaeovalleys supports recent models for palaeovalley formation that emphasize diachronous, cut‐and‐fill during falling stage and lowstands of relative sea level.

Paleomagnetic evidence for a post‐Eocene 90° CCW rotation of internal Apennine units: A linkage with Corsica‐Sardinia rotation?

AbstractWe report on an extensive paleomagnetic study (36 sites) of the Tuscan Nappe succession from the Northern Apennines Arc, aimed to reconstruct the tectonic evolution of the internal sector of this chain. We analyzed Eocene pelagic foreland ramp deposits (Scaglia Toscana Formation) and Oligocene–lower Miocene siliciclastic turbidites (Macigno and Falterona Formations). Paleomagnetic results show that the internal sector of the Northern Apennines underwent large counterclockwise (CCW) rotations with respect to the Adria‐Africa foreland. A decrease in the rotation magnitude was observed from the southern to the northern sector of the arc (from 91 to 36°). This trend is opposite to that observed in the more external units of Northern Apennines and demonstrates that the oroclinal bending model, which has been proposed for the external units of the chain, is not appropriate to explain the evolution of the internal sector of the arc. On the basis of the observed paleomagnetic pattern, we propose a new tectonic model in which the Tuscan and Falterona‐Cervarola units in the southern area were first rotated CCW along with the Corsica‐Sardinia block during its lower Miocene rotational drifting and were later involved in the main phases of rotational emplacement and translation toward the outermost sector (Umbria domain), thus yielding the final curved shape of the Northern Apennines chain. Data from this study represent the first paleomagnetic evidence of the influence of the Corsica‐Sardinia CCW rotation in the Apennines orogenic wedge deformation, in the general framework of the geodynamic evolution of the Central Mediterranean subduction system.

The Messinian Sicilian stratigraphy revisited: new insights for the Messinian salinity crisis

AbstractControversies around the Messinian salinity crisis (MSC) are because of the difficulties in establishing genetic and stratigraphic relationships between its deep and shallow‐water record. Actually, the Sicilian foreland basin shows both shallow and deep‐water Messinian records, thus offering the chance to reconstruct comprehensive MSC scenarios. The Lower Gypsum of Sicily comprises primary and resedimented evaporites separated in space and time by the intra‐Messinian unconformity. A composite unit including halite, resedimented gypsum and Calcare di Base accumulated between 5.6 and 5.55 Ma in the main depocentres; it records the acme of the Messinian Salinity Crisis during a tectonic phase coupled with sea‐level falls at glacials TG14‐TG12. These deposits fully post‐date primary gypsum, which precipitated in shallow‐water wedge‐top and foreland ramp basins between 5.96 and 5.6 Ma. This new stratigraphic framework results in a three‐stage MSC scenario characterized by different primary evaporite associations: selenite in the first and third stages, carbonate, halite and potash salt in the second one associated with hybrid resedimented evaporites.

The ‘Evaporiti di Monte Castello’ deposits of the Messinian Southern Apennines foreland basin (Irpinia–Daunia Mountains, Southern Italy): stratigraphic evolution and geological context

Abstract An evaporitic limestone–gypsum succession, belonging to the Evaporiti di Monte Castello Formation, is recognized in the late Messinian (Upper Miocene) of the Irpinia–Daunia Mountains in the Southern Apennines arc (Southern Italy). This unit is formed by diatomaceous marls, massive and laminated evaporitic limestones, and by primary and clastic gypsum. Detailed stratigraphical, sedimentological and strontium geochemistry data has permitted reconstruction of stratigraphic and facies relations of gypsum deposits, depositional environment and basin evolution. Genetically related gypsum lithofacies can be grouped into two facies associations. The autochthonous gypsum facies association consists of shallow water selenitic, acicular and laminated gypsum and is characterized by the absence of high-energy sedimentary structures. The redeposited clastic gypsum facies association consists of shallow- to deeper-water fine-grained laminated gypsum, gypsarenites, pebbly gypsarenites and gypsrudites, showing common features of resedimented deposits. Nodular structures occurring in the laminated gypsum lithofacies seem to be mostly related to late diagenetic processes. The sedimentary evolution during the evaporative phase was characterized by a gradual increase in salinity until gypsum precipitated; then the sedimentary conditions in the basin were characterized by almost homogenous salinity conditions, and influenced by events of gypsum reworking and resedimentation probably related to flooding episodes and local tectonic activity. The gypsum was deposited from mainly marine brines, based on their Sr isotopic compositions. This sedimentary series is an equivalent of the Lower Evaporites of other parts of the Mediterranean. The Messinian Monte Castello Formation evaporites represent an uncommon type of evaporitic succession, probably developed in a extensional setting in a basin located along the Apulian foreland ramp, in contrast with the northern Apennines and Sicilian basins (e.g. Vena del Gesso, and Caltanisetta basins), which are considered to be thrust-top basins of the Apennine–Maghrebian foreland basin system.

Pliocene–Pleistocene sequences bounded by subaerial unconformities within foramol ramp calcarenites and mixed deposits (Salento, SE Italy)

Nine discrete, metre-scale sequences, of Early Pliocene to Middle Pleistocene age, were deposited in the small Novoli graben (Salento peninsula, Puglia, S-Italy). They consist of carbonate and mixed carbonate-siliciclastic sediments, deposited on a slowly subsiding foreland ramp. Skeletal concentrations and intervening less fossiliferous intervals have been examined to provide information on major environmental parameters and infer the dynamics of their changes. Taphonomic and palaeoecological analyses indicate that storm-induced waves and currents, reduced sediment input, and settling behaviour of components were the main factors controlling the features of the various shellbed types. The concentrations were formed below fair-weather wave base in low-stress inner-to-outer shelf environments and are often associated with surfaces or intervals that are characterized by sedimentary condensation. Vertical change in the fossil content within individual cycles indicates water depth changes that were parallel to climatic fluctuations, hence may result from glacio-eustatic sea-level changes. Most sequences are bounded by subaerial, karstic unconformities. Because of the regional setting of low subsidence rate, the record of the relative sea-level fluctuations is incomplete. Episodes of subaerial exposure and concomitant effects of vadose diagenesis are documented by: (i) diagenetic changes leading to hardening of unconformity horizons; (ii) local subvertical solution pits developed in the vadose zone below unconformity surfaces; (iii) networks of polygonal cracks below unconformities; and (iv) infilling of solution pits and polygonal cracks with vadose silt and marine sediment inwashed during transgressions following the subaerial stages. Lower sequences are characterized by tighter cementation and significant increase in moldic and vuggy porosity, due to superimposition of the diagenetic effects of repeated high-amplitude sea-level fluctuations.

ABSTRACT: The Roles of Tectonics and Climate Change on The Carbonate Facies Architecture Development of an Active Foreland Ramp Margin: Evidence from The Madison Group, Southwest Montana

ABSTRACT: The Roles of Tectonics and Climate Change on The Carbonate Facies Architecture Development of an Active Foreland Ramp Margin: Evidence from The Madison Group, Southwest Montana

Foreland-basin sequence response to collisional tectonism

As structural salients and recesses evolved from reentrants and promontories along the collisional continental margin associated with the Taconic orogeny, cross- strike structural features provided a mechanism for transferring zones of relative subsidence and uplift across the Appalachian foreland basin. The regional distribution of Lower Silurian clastic sequences reflects this tectonic influence. Thick, aggradational sequences formed in areas corresponding to salients in response to high rates of sediment supply and creation of sediment accommodation. As the rate of sediment supply exceeded the rate of accommodation added, shoreline progradation onto the distal foreland ramp produced upward-coarsening sequences. In areas of structural recesses, accommodation was created by erosion during sea-level fall and lowstand. Upward-fining sequences formed as the topographic lows were filled during subsequent sea-level rise. Results from this investigation indicate that predictable variations in foreland-basin deposition and in the resulting stratigraphic pattern occur along regional tectonic strike as well as in the dip direction. The thickness of foreland-ramp sequences is greater in areas of salients than in recesses, whereas the ratio of sandstone to total thickness is greater in the recesses. Aggradational sequences grading laterally into upward-coarsening progradational sequences of the distal ramp characterize areas of relative subsidence, which provides a mechanism for creating sediment accommodation. In contrast, deep erosion, common unconformities, and incised valley fills are present in areas corresponding to recesses, where the rate of eustatic fall commonly exceeds the subsidence rate. These along-strike stratigraphic variations in response to collisional tectonism should be considered in the interpretation of other foreland-basin successions.

Recognition of Facies, Bounding Surfaces, and Stratigraphic Patterns in Foreland-Ramp Successions: An Example from the Upper Devonian, Appalachian Basin, U.S.A.

ABSTRACT A detailed stratigraphic study of the Upper Devonian Lock Haven Formation in the Appalachian basin provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the foreland-ramp setting. The common occurrence of graded beds, hummocky cross stratification, and shell lags indicates that storm processes played an important role in deposition. Gradational-based, coarsening-upward shoreface sequences of interbedded sandstone and mudstone formed during gradual fall in relative sea level. In contrast, sharp-based regressive sequences consist of laminated sandstone facies overlying a submarine erosion surface that developed in response to more rapid fall in relative sea level. Transgressive sequences become finer grained upward from conglomerate lag, which was deposited on a basal transgressive surface, to heterolithic facies and mudstone facies. A combination of eustasy and tectonic subsidence of the foreland ramp produced changes of relative sea level. These fluctuations provided a means for transporting sand onto the ramp and for producing repetition of stratigraphic patterns in the Lock Haven Formation. The Upper Devonian clastic succession, which includes both the Lock Haven Formation and the Catskill Formation, represents northwestward progradation of shoreline to nonmarine environments as sediment supply to the foreland ramp exceeded the rate of formation of accommodation. Compared to passive-margin shelves, thick fine-grained successions such as the Lock Haven Formation are more likely to form in foreland basins, where tectonic subsidence provides a mechanism for creating accommodation. Sediment influx from uplifted areas and absence of a slope break on the foreland ramp results in a higher rate of fine-grained clastic sedimentation than occurs on shelves of passive margins.

A Quaternary proximal foreland ramp and its continental fringe, Arabian Gulf, UAE

Abstract During Quaternary time, the southern flank of the Arabian Gulf can be considered as a classic, foreland basin ramp setting in which the sedimentological processes operating changed drastically along strike. The combined effects of sea-level fluctuations and wind, which have dominated the Quaternary coastal morphology and coastal evolution, are observed at a resolution rarely achieved in the stratigraphic record of palaeoramp sequences. The facies distributions and diagenetic characteristics of the relatively thin Pleistocene-Holocene strata provide valuable insights into the complex stratigraphy that can develop within an inner ramp and its continental fringe. In the UAE, Pleistocene sediments comprising mainly aeolian dunes overlie a deeply eroded topography of Upper Miocene strata. These sediments become increasingly carbonate rich and calcite cemented northwards towards the coast. Here they experienced severe aeolian deflation due to the later marine flooding of the aeolian sources on the formerly exposed areas lower down the ramp profile, and wind erosion along the coastal area because of a lack of sediment supply. The antecedent topography of Miocene and Pleistocene escarpments, mesas, islands, cemented seif dunes and coastal deflationary ‘Stokes surfaces’ provided a highly varied Holocene ramp margin undergoing coastal progradation (with offlap) during the 1–2 m relative sea-level fall of late Holocene time. Since then, marine carbonate deposition and evaporite diagenesis have dominated the Holocene sequence, but it is aeolian processes that largely control their ultimate distribution within the constraints of the contemporaneous sea-levels. Longshore currents, lateral and leeward accretion, infilling of stranded lagoons and deflation on a massive scale testify to the crucial role of wind in the development of this proximal ramp. The Holocene aeolian sediments, though extensive, are very different in character from the Pleistocene sediments. Two generations of Holocene supratidal anhydrites are recognized. The earlier one was partly transgressive, but the later one is regressive. In addition to the present-day oolith factories on the ebb tidal deltas at the seaward ends of tidal channels between the barrier islands of central Abu Dhabi, oolith factories also exist on exposed beaches and tidal embayments of western Abu Dhabi. They also occurred within Pleistocene and Holocene embayments. Flat-topped and sometimes shingled hardgrounds representing isochronous events extend over very large areas. So also do commonly observed iron oxide bands formed beneath the sabkha surfaces.

The Lower Miocene siliceous zone: a marker in the palaeogeographic evolution of the northern Apennines

The occurrence of coeval silica-rich and volcaniclastic layers within diverse tectonic domains of the northern Apennines provides the opportunity to discuss and redefine the old term “lithozone” in a modern light, showing its interest when used for stratigraphic correlation of genetically related deposits. Silica-rich horizons (bedded cherts and nodules) have been reported in the Lower Miocene strata of circum-Mediterranean countries as occurring at distinct stratigraphic levels. In the northern Apennines at least, they can be included in a single siliceous zone, representing the upper part of an unconformity-bounded depositional sequence. The siliceous zone is characterized by a reduced rate of sedimentation, with the local aspect of a condensed sequence, and the almost ubiquitous presence of lithologic markers: cherts, ash layers, bentonites, and locally glaucony. None of them can be considered distinctive of the zone, but their assemblage is. The local intercalation of turbidite siliciclastics complicates the picture, especially the location of the lower boundary of the zone. These terrigenous units form the lower part of the sequence and dilute the overlying siliceous and volcanogenic horizons, but do not mask them completely. Turbidites cause a thickening of the sequence and of the intervals between the markers, up to two orders of magnitude in the main foredeep area where the Cervarola and Falterona Formations were deposited. Smaller turbidite bodies occur in the semi-allochthonous basins located onto the thrust belt (Antognola Formation), whereas the ramp connecting the foredeep and foreland to the east is virtually turbidite-free (Bisciaro Formation). In semi-allochthonous and foredeep depocentres, both sand input and frequence of resedimentation events decrease upwards within the siliceous zone, concurrently with a concentration of chert and volcaniclastic layers. Carbonate or mixed carbonate-siliciclastic megaturbidites are interbedded in the more shaly, upper sections of the Cervarola and Falterona Formations and are probably correlatable with calcareous tempestites (or relatively shallow-water turbidites) of the Bisciaro area. The siliceous zone has a geochemical signature, too, showing a silica enrichment, which is not accompanied by major or minor elements concentrations. This supports the hypothesis that silica is related more directly to biological activity than to volcanic events. Abrupt facies variations from deep-water turbidite/hemipelagite to shelf deposits, concurrently with a change in the terrigenous supply, occur at the top of the zone, allowing its interpretation as a sequence boundary. This boundary is marked by obvious unconformities both in the semi-allochthonous basins and in the foredeep, laterally passing into correlative conformable surfaces in the foreland ramp. The regional significance of the siliceous zone and its possible use as a stratigraphic marker over a great areal extent suggest using a single name for this unit instead of the chaotic terminology which has grown up in the past. The evidence of a pronounced tectonic (Burdigalian) phase that caused a substantial rearrangement of all orogenic basins at the upper zone boundary, supports the idea that the siliceous zone marks a divide in the tectono-sedimentary evolution of the Apenninic orogenic system.

Paleomagnetic study of thrust-sheet motion along the Rocky Mountain front in Montana

Near Wolf Creek, Montana, the fold and thrust belt intersects the central Montana trough at the northern edge of the Helena salient. Remanent magnetizations from 18 of 31 paleomagnetic sites in the Late Cretaceous Two Medicine Formation (∼79 Ma) document counterclockwise rotations of thrust-bound rocks along the margin of the salient. Results from 12 reliable sites in the upper of two thrust sheets gives a mean direction of Dec = 328.4°, Inc = 67.9°, α 95 = 5.63°. Results from 6 reliable sites in the underlying thrust sheet yield Dec = 321.9°, Inc = 69.3°, α 95 = 11.87°. The difference in these directions is not significant at the 95% confidence level. The thrust sheets show no relative rotation. Combining all 18 sites gives an average direction of Dec = 326.3°, Inc = 68.4°, α 95 = 4.94°. This average direction is 25.2° ± 14.0° counterclockwise from a Late Cretaceous reference direction (Dec = 351.5°, Inc = 69.3°) established from the adjacent Adel Mountain volcanic rocks (sim;76 Ma). Comparison to an older (∼88 Ma) pole established from the Niobrara Formation indicates 11.7° ± 11.4° of counterclockwise rotation. The paleomagnetic results, with other geologic evidence, show that both thrust sheets interacted with an oblique, lateral foreland ramp.

Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile

Research Article| January 01, 1991 Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile TERRY J. WILSON TERRY J. WILSON 1Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 Search for other works by this author on: GSW Google Scholar Author and Article Information TERRY J. WILSON 1Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (1): 98–111. https://doi.org/10.1130/0016-7606(1991)103<0098:TFBATF>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation TERRY J. WILSON; Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile. GSA Bulletin 1991;; 103 (1): 98–111. doi: https://doi.org/10.1130/0016-7606(1991)103<0098:TFBATF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract During the Mesozoic evolution of the southernmost Andes, back-arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting. A stratigraphic record of this tectonic transition is described from Jurassic and Cretaceous rock units exposed in the Patagonian fold-thrust belt at 51°S latitude. Initiation of an extensional deep-marine trough in Late Jurassic time is documented by interstratified rhyolites and marine mudstones in the Tobifera Formation. Development of the Early Cretaceous Rocas Verdes back-arc basin is recorded by Zapata Formation submarine-slope deposits, which draped the west-facing, passively subsiding cratonic margin of the basin. The onset of Andean compressional orogenesis and creation of the Magallanes foreland basin is marked by the influx of coarse-grained sandstone turbidites of the Albian-Cenomanian Punta Barrosa Formation. During Late Cretaceous time, the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east. This geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back-arc basin margin beneath the load of the obducted Rocas Verdes basin floor. Changes in depositional regime and sediment dispersal patterns in the latest Cretaceous, together with an eastward shift in the Magallanes basin depocenter, record cratonward migration of deformation in the Patagonian fold-thrust belt. Loading of the thicker, cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.