Forebulge Unconformity Research Articles

Foreland basin development in response to Proto-Tethyan Ocean closure, western North China Block

Closure timing of the northeastern Proto-Tethyan Ocean between the Qilian-Qinling Terranes and the North China Block remains unsolved, with assumptions ranging from end-Ordovician to the Devonian. To address this issue, integrated studies of stratigraphy and geochronology were conducted on Late Ordovician strata in the southwestern Ordos, which recorded the earliest tectonic transition from passive margin to foreland basin in the westernmost North China Block. Stratigraphic and paleontologic syntheses demonstrate that pre-Katian strata are shallow-marine deposits dominated by benthonic faunas. Meanwhile, Katian successions above a paraconformity are characterized by deep-water debrites and turbidites containing abundant planktonic graptolites. Provenance analysis reveals an evolving source from the North China Block basement to the Qilian-Qinling arc terranes at the beginning of Katian (ca. 450 Ma). Detrital zircons from pre-Katian quartz arenites yield ages of ca. 1600–2800 Ma, significantly older than their depositional timing. In contrast, Katian turbidites in the southwestern Ordos and the North Qilian Orogen display similar age patterns dominated by ca. 450–900 Ma ages. These clues imply a basin-filling shift from passive margin to underfilled foreland, separated by forebulge unconformity formation at Sandbian-Katian boundary. Stratigraphic correlation further suggests that sequence of foreland basin-infill transitions was broadly simultaneous along strike more than ~1200 km across the southwestern North China Block, including forebulge flexural uplift, followed by rapid shallow shelf drowning to abyssal plain, until arrival and superimposition of arc-derived turbidites. Along-strike synchronicity of orogenic activities implies that closure of the northeastern Proto-Tethys was nearly synchronous. The L-shaped orogen-parallel foreland encompassing the southwestern North China Block unveils oroclinal bending of the Qilian-Qinling collision belt, which still dominates the geology of present northeastern Tibet. Our new insights provide a stratigraphic constraint for the timing and mode of the initial elimination of the northeastern Proto-Tethys.

Geochemical records in subaerial exposure environments in Croatia using discriminant function analysis of bauxite data

Croatian bauxites are long known for their chemical and physical diversity arisen from their characteristic origin and emplacement within the area of the Adriatic–Dinaric carbonate platform (ADCP). They include eight horizons spanning the period between the Upper Triassic (Carnian) and the Miocene, formed on subaerially exposed platform paleoenvironments. The bauxite genesis is recorded in the bauxite geochemical composition as a unique signature of tectonostratigraphic evolution of the different parts of the Croatian Karst, including, for example, the forebulge unconformity typical for the Istrian area. In this work, an explanation of the typical patterns of bauxite formation is based on the construction of a discriminant function model (DFM) resulting from the compositional data (CoDa) analysis of bauxite geochemical data (major and trace elements). The model shows that the greatest part of the variation contained in the analyzed bauxite data (the first discriminant function, DF1) is associated with systematic alteration of geochemical composition in time, emphasizing characteristic decrease in clay component and gradual enrichment in heavy metals from oldest (Upper Triassic) to the youngest (Miocene) bauxites. In the general scheme, particular bauxite horizons represent standalone groups (Upper Triassic) while others form clusters showing increase and/or decrease of a particular set of elements signaling the changes in environmental conditions during the considered geological history of ADCP. Other discriminant functions (DF2 and DF3) also contribute to the all-inclusive distinction between the eight a priori defined bauxite groups discriminated by the characteristic set of geochemical variables where DF2 typically refers to the process of desilication, while DF3 to that of deferralitization.

Forebulge migration in the foreland basin system of the central‐southern Apennine fold‐thrust belt (Italy): New high‐resolution Sr‐isotope dating constraints

AbstractThe Apennines are a retreating collisional belt where the foreland basin system, across large domains, is floored by a subaerial forebulge unconformity developed due to forebulge uplift and erosion. This unconformity is overlain by a diachronous sequence of three lithostratigraphic units made of (a) shallow‐water carbonates, (b) hemipelagic marls and shales and (c) siliciclastic turbidites. Typically, the latter two have been interpreted regionally as the onset of syn‐orogenic deposition in the foredeep depozone, whereas little attention has been given to the underlying unit. Accordingly, the rate of migration of the central‐southern Apennine fold‐thrust belt‐foreland basin system has been constrained, so far, exclusively considering the age of the hemipelagites and turbidites, which largely post‐date the onset of foredeep depozone. In this work, we provide new high‐resolution ages obtained by strontium isotope stratigraphy applied to calcitic bivalve shells sampled at the base of the first syn‐orogenic deposits overlying the Eocene‐Cretaceous pre‐orogenic substratum. Integration of our results with published data indicates progressive rejuvenation of the strata sealing the forebulge unconformity towards the outer portions of the fold‐thrust belt. In particular, the age of the forebulge unconformity linearly scales with the pre‐orogenic position of the analysed sites, pointing to an overall constant migration velocity of the forebulge wave in the last 25 Myr.

Constraining the onset of flexural subsidence and peripheral bulge extension in the Miocene foreland of the southern Apennines (Italy) by Sr-isotope stratigraphy

In fold and thrust belts developing at convergent margins, the migration of the advancing wedge is accompanied by bulging of the downgoing plate, followed by the development of a foredeep basin filled by a thick succession of syn-orogenic sediments. The transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system. In deep water environments, the record of this transition is typically complete and progressive. Conversely, in the shallow-water/continental environment of many collisional systems, the uplift of the forebulge area can imply emersion and erosion, obliterating the stratigraphic record of key steps of the evolution of the orogenic system. The southern Apennines constitute one of these collisional fold and thrust belts where the development of the forebulge has implied emersion and erosion, with the development of a Miocene forebulge erosional unconformity, accompanied by extensional deformation associated with the bending of the lithosphere during the forebulge stage. In this paper, we use strontium isotope stratigraphy to constrain with unprecedented time-resolution the age of the forebulge unconformity in areas presently incorporated in the northern sector of the southern Apennines fold and thrust belt. Integration of our results and those of previous studies indicates, at the regional scale, a younging toward the foreland of the forebulge unconformity across the belt. Our high-resolution ages also reveal a diachronous onset of the flexural subsidence over short distances, associated with the occurrence of horst and graben structures, possibly resulting from inherited paleotopography along with forebulge extension. This work highlights how high-resolution dating is critical to unravel the evolution of foreland basin systems at different scales.

The foreland state at the onset of the flexurally induced transgression: data from provenance analysis at the peripheral Carpathian Foredeep (Czech Republic)

Abstract The Žerotice Formation recognised in a confined area NE–SE of Znojmo represents a basal member of the sedimentary succession of the southwestern margin of the Carpathian Foredeep in Moravia (Czech Republic). Two facies associations were recognised within the formation. The first one mantles the pre-Neogene basement with an irregular unconformity, reflects arid climatic conditions and deposition of episodic shallow, high-energy stream flows and/or mass flows (alluvial to fluvial deposits). The second facies association is interpreted as lagoonal to distal flood plain deposits. The barren unfossiliferous deposits of the Žerotice Formation are covered by nearshore marine Eggenburgian deposits. The boundary between these deposits represents a sequence boundary (i.e. the basal forebulge unconformity). Detailed provenance studies of successive beds below and above this sequence boundary showed differences in the source area and paleodrainage. Both the local primary crystalline rocks (Moravian and Moldanubian Unit, Thaya Batholith) and older sedimentary cover (especially Permo–Carboniferous sedimentary rocks) form the source of the Žerotice Formation. All these geological units are located only a few km away from the preserved areal extent of the deposits of the Žerotice Formation (short transport and a local source). The source areas of the overlying marine Eggenburgian beds are located far more to the W and NW in the Moldanubian and Moravian Units (longer transport, extended source area). Local confined preservation of the Žerotice Formation is preliminarily explained as connected with a tectonically predisposed paleovalley.

First-order foreland cycles: Interplay of flexural tectonics, dynamic loading, and sedimentation

Foreland systems on overriding plates (i.e., retroarc settings) form through the deflection of the lithosphere in response to a combination of supra- and sublithospheric loads. Supracrustal loading by orogens leads to the partitioning of foreland systems into foredeep, forebulge, and back-bulge flexural provinces. Thrusting in the orogenic belt results in foredeep subsidence and forebulge uplift, and the reverse occurs as orogenic load is removed by erosion or extension. This pattern generates out-of-phase ('reciprocal') stratigraphies across the flexural hingeline that separates the foredeep from the forebulge. Coupled with flexural tectonics, accommodation is also modified by dynamic (sublithospheric) loading. The latter mechanism operates at larger, continental scales, with rates controlled by the velocity and the angle of subduction.The interplay of accommodation and sediment supply during the lifespan of a foreland system controls the long-term shift from underfilled to overfilled accommodation conditions, which defines the first-order foreland cycle. Predictable shifts in the balance between flexural tectonics and dynamic loading during the evolution of a foreland system allow the subdivision of the first-order foreland cycle into early and late stages dominated by flexural tectonics, separated by an intermediate stage dominated by system-wide dynamic subsidence. The early stage is defined by an underfilled foredeep that hosts a deep-water environment, and an exposed forebulge whose progradation and erosion generates the ‘forebulge unconformity' at the base of the first-order foreland sequence. Submergence of the forebulge during the intermediate stage establishes an interior seaway across the entire foreland system. The late stage marks a shift to a system-wide continental setting, which defines the overfilled phase of the foreland cycle.

Migration of the carbonate ramp and sponge buildup driven by the orogenic wedge advance in the early stage (Carnian) of the Longmen Shan foreland basin, China

Abstract The marine Carnian the Maantang Fm. overlies a flexural forebulge unconformity and records the initial establishment, drowning and migration of a carbonate ramp and sponge buildup along the forebulge margin of the Longmen Shan foreland basin. The Maantang Fm. is of wedge-shaped geometry, and is composed of oolitic and bioclastic limestones, siliceous sponge reef and shale in an upward-fining succession. The formation shows the establishment and drowning of a distal margin carbonate ramp and sponge buildup, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics. The formation also shows the transition from shale cratonward into carbonate rock southeastward. The sponge reefs and shoal were deposited on a carbonate ramp on the distal margin of the early foreland basin. The growth rate of sponge reefs is 0.04 mm/yr, equivalent to the rate of relative sea level rise of 0.01–0.05 mm/yr. The sponge buildup and oolitic shoal are divided into seven zones southeastward on the carbonate ramp along the basal unconformity. Their migration rate of 18 mm/yr from NW to SE coincides with the estimated orogenic wedge advance rate (5–15 mm/yr, Li et al., 2003), a clear indication that the advancing wedge controlled the migration rate of foreland oolitic shoal–siliceous sponge reef. We have inferred that the tectonic load of the Longmen Shan orogenic wedge led to flexural subsidence and rising relative sea level in the foreland basin located at the western margin of the Yangtze Craton, driving the growth and subsequent drowning of the oolitic shoal–sponge buildup in the early stage of the foreland basin. We propose that the drowning and migration processes were the sedimentary response to the orogenic wedge advance toward the Yangtze Craton, and to the rapid closure of the Carnian Songpan–Ganzi remnant ocean basin.

Evolution of the Longmen Shan Foreland Basin (Western Sichuan, China) during the Late Triassic Indosinian Orogeny

ABSTRACTThe Longmen Shan Foreland Basin developed as a flexural foredeep during the Late Triassic Indosinian orogeny, spanning the time period c. 227–206 Ma. The basin fill can be divided into three tectonostratigraphic units overlying a basal megasequence boundary, and is superimposed on the Palaeozoic–Middle Triassic (Anisian) carbonate‐dominated margin of the South China Block. The remains of the load system responsible for flexure of the South China foreland can be seen in the Songpan‐Ganzi Fold Belt and Longmen Shan Thrust Belt. Early in its history the Longmen Shan Foreland Basin extended well beyond its present northwestern boundary along the trace of the Pengguan Fault, to at least the palinspastically restored position of the Beichuan Fault.The basal boundary of the foreland basin megasequence is a good candidate for a flexural forebulge unconformity, passing from conformity close to the present trace of the Beichuan Fault to a karstified surface towards the southeast. The overlying tectonostratigraphic unit shows establishment and drowning of a distal margin carbonate ramp and sponge build‐up, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics, collectively reminiscent of the Alpine underfilled trinity of Sinclair (1997). Tectonostratigraphic unit 2 is marked by the severing of the basin's oceanic connection, a major lake flooding and the gradual establishment of major deltaic‐paralic systems that prograded from the eroding Longmen Shan orogen. The third tectonostratigraphic unit is typified by coarse, proximal conglomerates, commonly truncating underlying rocks, which fine upwards into lacustrine shales.The foreland basin stratigraphy has been further investigated using a simple analytical model based on the deflection by supracrustal loads of a continuous elastic plate overlying a fluid substratum. Load configurations have been partly informed by field geology and constrained by maximum elevations and topographic profiles of present‐day mountain belts. The closest match between model predictions and stratigraphic observations is for a relatively rigid plate with flexural rigidity on the order of 5 × 1023 to 5 × 1024 N m (equivalent elastic thickness of c. 43–54 km). The orogenic load system initially (c. 227–220 Ma) advanced rapidly (>15 mm yr−1) towards the South China Block in the Carnian, associated with the rapid closure of the Songpan‐Ganzi ocean, before slowing to < 5 mm yr−1 during the sedimentation of the upper two tectonostratigraphic units (c. 220–206 Ma).

Forebulge migration in the Cretaceous Western Interior basin of the central United States

ABSTRACT This study combines stratigraphic evidence with geodynamic modelling to demonstrate that a forebulge played an identifiable role in Cenomanian–Turonian erosion and sediment accumulation in the North American Western Interior basin. The early to middle Turonian forebulge migrated progressively eastwards, and by the upper middle Turonian acted as a ‘backstop’ against which barrier islands formed in the axial basin.This paper focuses on the progressive migration of an unconformity on the forebulge. The lengthwise orogen‐parallel orientation and time‐transgressive orogen‐normal migration of the forebulge unconformity are characteristics that differentiate it from unconformities developed on reactivated basement structures. We present a conceptual model in which the unconformity formed as the seafloor was uplifted by forebulge‐related flexure to a water depth at which submarine bypass and erosion occurred. A numerical model that describes forebulge migration in response to load dispersal by erosion of the orogenic front and sedimentation into the foredeep indicates that the distance from the thrust front to the forebulge is within reasonable bounds established using a flexural rigidity of 3×1024 Nm.We identify architecturally similar, coeval unconformities from Montana to New Mexico, and interpret the similar distance from the thrust front to a point where each unconformity dissipates as indicative of a uniform lithospheric flexural response along the orogenic front. Here we ascribe cratonward (west‐to‐east) forebulge migration to erosional load redistribution, whereas orogen‐parallel (north–south) stratigraphic climb of the forebulge unconformities developed in response to depocentre migration. Inherited lithospheric inhomogeneities may have allowed the forebulge in central Colorado to crest farther from the orogen than to the north and south.

Flexural partitioning of the Late Archaean Witwatersrand foreland system, South Africa

The Witwatersrand Basin is part of a retroarc foreland system formed during the Late Archaean in response to crustal thickening along the northern margin of the Kaapvaal Craton. This foreland system may be partitioned into foredeep, forebulge, and back-bulge flexural provinces. The preserved sedimentary fill occupies the region of the flexural foredeep, and displays a typical transition from underfilled (deep and shallow marine) to overfilled (fluvial and alluvial fan) phases. Loading in the Witwatersrand thrust belt took place in relation to the oblique convergence of two compressional stress fields. As a result, the foredeep is restricted along the strike to a distance of <400 km, and the forebulge is constrained at the intersection between the areas of influence of the two stress fields. The apex of the forebulge is outlined by the gravity line of −130 mgal, which matches the shape of the theoretical depositional limit of the foredeep. The back-bulge region of the foreland system is superimposed on the divergent continental margin of the Kaapvaal Craton, and hosts the Mozaan Group of the Pongola sequence. The Mozaan and Witwatersrand successions are correlative and display in-phase stratigraphies, as being part of the same foreland system; the gap between their areas of occurrence may be related to the uplift and erosion in the forebulge region. The forebulge migrated a relatively short distance down dip (i.e., <100 km), causing the erosion and wedging out of the underlying Dominion Group. This erosional surface displays all the diagnostic features of a basal (forebulge) unconformity, with an increasing stratigraphic gap towards the foreland, and a correlative conformity in the more proximal parts of the basin. The Witwatersrand foreland system formed on a newly cratonized continental lithosphere, which gave the basin a small flexural wavelength (i.e., ∼490 km from the center of orogenic loading to the axis of the back-bulge) relative to most Phanerozoic counterparts. The forebulge unconformity may be associated with the lack of dynamic loading, which is probably a common feature for all Precambrian retro-foreland systems due to the lower rates of subduction associated with the early stage of plate tectonic processes.

Characteristics and development of the flexural forebulge and basal unconformity of Western Taiwan Foreland Basin

The Western Taiwan Foreland Basin formed during the Early Pliocene as the flexural response to the loading of Taiwan orogen on the Eurasian plate. Using more than 4000km of marine multi-channel seismic reflection profiles and 22 exploration wells offshore and on land, we recognized a major unconformity separating the overlying sediments of foreland basin from the underlying passive margin strata and a flexural forebulge located at the distal part of the foreland basin. Those allow us to refine and extend the area of the Western Taiwan Foreland Basin to the middle of the Taiwan Strait. The Western Taiwan Foreland Basin is about 350km long and 150km wide between the west front of the Taiwan orogen and the Chinese craton.Seismic lines show that a narrow zone of uplifted Miocene passive margin sequences at the distal parts of the foreland basin extends approximately parallel to the strike of the deformation front of the Taiwan orogen. The Miocene bulge is generally 200m high, a subtle morphologic/structural feature, and is characterized by an unconformity where Quaternary sediments overlie the eroded Miocene strata. Positive Bouguer gravity anomalies associated with the bulge on the cratonic side suggest that the Chinese continental margin is flexed upward to produce the bulge with higher density materials compared to the negative gravity anomalies of the sediments deposited in the adjacent foreland basin.The base of the foreland basin fills is a regional angular unconformity. The stratigraphic patterns of sediments overlying the unconformity change laterally, towards the Taiwan orogen, into conformity or disconformity of insignificant gap at the coastal areas. In contrast, the chronostratigraphic gap across the unconformity increases cratonward, with the largest gap at flexural forebulge. The increase in stratigraphic gap reflects the progressively younger transgression of the Pliocene–Quaternary orogenic sediments from Taiwan onto the Chinese passive margin combined with progressively older Miocene units subcropping towards the forebulge. The characteristic geometry, stratigraphic pattern and large gap up to 10Ma suggest that the basal unconformity is a flexural forebulge unconformity rather than a type 1 unconformity, mainly due to sea level change.The flexural forebulge and associated basal unconformity mark transition from shelf-slope-rise deposition on a passive margin to deposition in a foreland basin associated with a convergent margin. The flexural unconformity developed during the collision between the Luzon Arc and the Chinese continental margin, reflecting the uplift and erosion of progressively cratonward migrating forebulge.

Eocene‐early Miocene foreland basin development and the history of Himalayan thrusting, western and central Nepal

Sedimentologic, petrographic, and U‐Pb detrital zircon ages from middle Eocene through early Miocene sedimentary rocks in the Lesser Himalayan zone of western and central Nepal indicate that a peripheral foreland basin system had developed in the eastern Himalayan collision zone by middle Eocene time. The shallow‐marine, Eocene Bhainskati Formation accumulated in a back‐bulge depozone between a southward migrating forebulge and the Indian craton. Migration of the forebulge through this region during Eocene‐Oligocene time produced a regional unconformity that spans ∼15–20 Myr. By early Miocene time, the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone, represented by fluvial deposits of the Dumri Formation. Continued southward migration of the foredeep during the Neogene accommodated the fluvial Siwalik Group. Light mineral provenance data and U‐Pb detrital zircon ages suggest that the Bhainskati was derived partly from Tethyan sedimentary rocks of the Tibetan Himalayan zone during initial growth of the Himalayan fold‐thrust belt. The Dumri was derived from metasedimentary and crystalline rocks of the Greater Himalayan zone during emplacement of the Main Central thrust and contemporaneous tectonic unroofing by normal faulting along the South Tibetan detachment system. The Lesser Himalayan crystalline thrust sheets were emplaced soon after deposition of the Dumri Formation, ∼15–10 Ma. Paleocurrent and lithofacies data from the Dumri Formation indicate deposition by west‐southwestward flowing rivers that drained into the Indus portion of the Himalayan foreland basin system during the early Miocene. Thick channel sandstones in the lower Dumri may represent the early Miocene counterpart of the modern Ganges River. Eastward diversion of the Ganges drainage system to near its present location had occurred by ∼15 Ma, as the high‐standing Aravalli Range on the northern Indian shield approached the front of the fold‐thrust belt. Assuming reasonable values for the flexural rigidity of Indian lithosphere, the time span of the forebulge unconformity yields a velocity of ∼14–33 mm/yr for the southward migration of the fold‐thrust belt relative to India. This range of values is consistent with Neogene and present‐day estimates and suggests that only one third to one half of India‐Eurasia convergence has been accommodated by shortening in the Himalayan fold‐thrust belt since the onset of collision.

Recognition of Forebulge Unconformities Associated with Early Stage Foreland Basin Development: Example from the North Alpine Foreland Basin

The peripheral upwarp (forebulge) associated with early foreland basin evolution may be recorded as an unconformity at the base of the foreland basin fill. A forebulge unconformity represents a distinctive type of megasequence boundary. Forward modeling of foreland flexure illustrates the expected geometry of an unconformity cut by erosion of the forebulge during the early stages of foreland basin development: internal conformity, a zone of rapid increase in the depth of erosion, and an external zone of steady-state stratigraphic gap. This geometry is modified by the preexisting bathymetry and by eustatic sea level changes, which result in the formation of a composite unconformity. However, the geometry of the unconformity and the onlap of the overlying sediments are fund mentally different from those associated with a major eustatic sea level fall, and represent a distinctive assemblage of petroleum play components. Producing hydrocarbon accumulations are associated with both forebulge arching and truncational unconformity traps. Modeling achieves an accurate simulation of the geometry and stratigraphic gap of the basal unconformity of the North Alpine foreland basin (Switzerland). Further support for the flexural forebulge interpretation is provided by facies patterns in the overlying sediments. Suppression and smothering of forebulges during the late (Molasse) stage of foreland basin development, coupled with the cannibalization of the early (Flysch) stage stratigraphy by the advancing orogenic wedge, may limit the formation and recognition of forebulge unconformities. Model results may be helpful in predicting the timing, duration, and amount of uplift associated with flexural forebulges, and thereby aid in assessing petroleum play potential, particularly the formation of traps and the development of enhanced porosity.