Unconformity-bounded Sequences Research Articles

Paleogeography of Late Eocene to earliest Miocene Te Kuiti Group, central-western North Island, New Zealand

We present a series of 13 paleogeographic maps representing development of the Waikato–King Country Basin through the Late Eocene to Early Miocene in central-western North Island when the New Zealand platform was undergoing widespread marine inundation. The maps are the end-point of a basin analysis of the Te Kuiti Group, which has included development of a revised lithostratigraphy, biostratigraphy, chronostratigraphy and the application of facies analysis. The new stratigraphic framework has identified six major unconformity-bound sequences within the Te Kuiti Group and the paleogeographic maps are drawn for time-slices through unconformities and systems tracts. A major unconformity between the Whaingaroa Formation and the Aotea Formation dated c. 29 Ma marks the start of reverse faulting on the Taranaki Fault. At c. 27 Ma, reverse displacement on the Manganui Fault started. Several phases of displacement on the Manganui Fault ensured that land persisted over part of the Herangi High throughout the Duntroonian (Ld) and Waitakian (Lw) Stages spanning the Late Oligocene and earliest Miocene.

Sea-level control of New Jersey margin architecture: Palynological evidence from Integrated Ocean Drilling Program Expedition 313

Integrated Ocean Drilling Program Expedition 313 recovered Miocene sequences at Holes M0027A and M0029A on the New Jersey shallow shelf that contain a characteristic acid-resistant organic component. The palynofacies within each sequence reflects variations in terrigenous versus authigenic flux through the Miocene that are associated with sea-level change. Very high ratios of terrigenous versus marine palynomorphs and of oxidation-resistant versus susceptible dinocysts are associated with seismic sequence boundaries, consistent with their interpretation as sequence-bounding unconformities generated at times of low sea level. Comparison of palynological distance from shoreline estimates with paleodepth estimates derived from foraminiferal data allows relative sea level to be reconstructed at both sites. Ages assigned using dinocyst biostratigraphy are consistent with other chronostratigraphic indicators allowing sequence boundaries to be correlated with Miocene oxygen isotope (Mi) events. Paleoclimatic evidence from the pollen record supports the global climate changes seen in the oxygen isotope data. Although chronological control is relatively crude, Milankovitch-scale periodicity is suggested for parasequences visible in thick sequences deposited in relatively deep water where substantial accommodation existed, such as during the early Langhian at Site 29 (Middle Miocene Climatic Optimum). Palynological analysis thus supports the long-held hypothesis that glacioeustasy is a dominant process controlling the architecture of continental margins.

Upper Mississippian (Lower Carboniferous) carbonate stratigraphy and syndepositional faulting reveal likely Ouachita flexural forebulge effects, eastern Kentucky, U.S.A.

Stratigraphic analysis of late Middle to early Late Mississippian (Early Carboniferous; late Meramecian–early Chesterian) carbonates in east-central and northeastern Kentucky, U.S.A., shows six unconformity-bound sequences. The earliest, late Meramecian, St. Louis sequence shows a relatively uniform distribution and a typical eastward Appalachian gradient along the ENE-trending outcrop belt on the western margin of the Appalachian Basin. However, abrupt uplift and exposure of deeper water St. Louis deposits, indicated by erosional thinning and well-developed exposure features, at the Meramecian–Chesterian transition signal inception of regional early Chesterian uplift that intensively altered the nature and distribution of the five overlying early Chesterian carbonate sequences. Uplift was related to the syndepositional reactivation of WNW-trending basement faults that broke up the St. Louis platform and generated a mosaic-like distribution of the overlying five sequences due to structurally related erosional incision and paleoslope change to the southwest. These early Chesterian events coincide with a major phase of northeast-directed Ouachita Orogeny on the southern margin of Laurussia and concomitant forebulge migration to the ENE. Modeling of regional stress distribution, as well as timing and location during bulge migration, strongly indicate that a migrating Ouachita bulge reactivated basement structures and generated the noted stratigraphic anomalies in eastern Kentucky and nearby areas. The likelihood of low-amplitude flexural waves, like the Ouachita bulge, migrating across areas where they were not previously considered, has major implications for understanding structural, stratigraphic, and eustatic anomalies in supposedly stable cratonic areas far removed from active tectonism.

Regional Sequence Stratigraphy, Biostratigraphy, and Facies of the Upper Cretaceous Austin Chalk in South and Central Texas

This regional study constructs a sequence stratigraphic framework for the Upper Cretaceous Austin Chalk in South and Central Texas, based on new nannofossil biostratigraphy, core descriptions, correlations of wireline logs, and synthesis of outcrop, core, and paleontological descriptions from literature. The study area extends along strike from the Rio Grande to Austin, and along dip from the outcrop belt to the limit of downdip well control at the Lower Cretaceous shelf edges. New biostratigraphic control enables correlations across the region and ties the stratigraphic nomenclature from the outcrop to the major producing petroleum fields on either side of the San Marcos Arch. The upper Turonian–lower Campanian Austin Chalk consists of three unconformity-bounded sequences, with a basal unconformity, two internal unconformities, and an overlying unconformity. Sequence boundaries are recognized by nannofossil biostratigraphic hiatuses, which coincide with hardgrounds, Glossifungites ichnofacies, and coarse-grained glauconitic and phosphatic skeletal lags in cores and outcrops. Glauconite-rich beds have regionally extensive log character that overlie angularly truncated section. Facies within sequences can be distinguished as inner ramp (light colored skeletal wackestone, with diverse burrowers and macrofossils), middle ramp (medium gray horizontally burrowed skeletal wackestone), and outer ramp (interbedded dark horizontally burrowed and laminated skeletal wackestone), based on a depositional model of increasing oxygen content in shallower water. Regional cross sections show that the three sequences (AC–I to AC–III) in the Austin Chalk have an extremely asymmetric distribution across the San Marcos Arch. The upper Turonian–lower Coniacian section (AC–I) is very thick west of the San Marcos Arch but is almost completely eroded in the east. The upper Coniacian–Santonian section (AC–II) is very thick east of the San Marcos Arch but is almost completely eroded over the San Marcos arch, and is thin on the west side. The upper Santonian–lower Campanian sequence (AC–III) is more balanced, but similar to the middle sequence. Downdip, the Austin Chalk thins dramatically toward the relict Lower Cretaceous Edwards margin. The Austin Chalk was deposited as a downdip thinning wedge, due to greater carbonate productivity updip and non-deposition or erosion downdip. This interpretation is based on the geometry of the overlying Anacacho Formation, which thickens as the Austin Chalk thins, and internal facies relationships and markers within the Austin Chalk. Petroleum reservoirs in the Austin Chalk occur across a range of inner to outer ramp depositional environments but generally are older on the west side of the San Marcos Arch compared to the east side.

Unraveling the “Eaglebine”: A Sequence Stratigraphic Framework for the Eagle Ford Group in the East Texas Basin

A major stratigraphic problem presently exists in the southern East Texas Basin. Geoscientists working in the subsurface in this region are no longer sure, as to what strata, between the Buda Formation and Austin Group, are equivalent to the Woodbine and Eagle Ford groups, as defined along the outcrop belt. As a result, this stratal succession is now commonly just referred to as the “Eaglebine.” To resolve this dilemma, a regional grid of well-log cross-sections, tied to petrophysical and geochemical data from (1) two shallow subsurface government research cores and (2) a deeper subsurface industry core, was used to unravel the “Eaglebine.” This research revealed that the “Eaglebine” in the southern East Texas Basin actually consists of a vertical succession four petrophysically and geochemically distinct unconformity-bounded sequences (sequences 1 to 4, from the base up), all of which (1) have a sharp basal contact and (2) display underlying stratal terminations, on the grid of regional well-log cross-sections. The lowermost “sequence 1,” which is interpreted as the False Buda, is a moderate gamma ray (GR)/resistivity unit, that is rich in Ca, but poor in total organic content (TOC), Al, and Ti. “Sequence 2,” which is interpreted as the Pepper Formation (Woodbine Group), is a low-resistivity unit, that is poor in Ca and TOC, but rich in Al and Ti. “Sequence 3,” which is interpreted as the Lower Eagle Ford Formation (LEF), is a high-resistivity/GR unit, that overall is enriched in Ca and TOC, but poor in Al and Ti. The uppermost unit, “Sequence 4,” which is interpreted as the Lower Member of the Upper Eagle Ford Formation (UEF), is primally a low-resistivity unit, that is rich in Al and Ti, but overall poor in Ca and TOC. In general, across the study area, Ca– and TOC–rich, high-resistivity LEF mudstones separate more Al–rich, as well as Ca– and TOC–poor strata, of both the Woodbine Group/Freestone delta (below) from the UEF/Harris delta (above).

Paleogeographic and tectonic evolution of the Pranhita-Godavari Valley, Central India: a Stratigraphic perspective

The Pranhita-Godavari Valley of central India preserves records of repeated opening and closing of Proterozoic and Gondwana rifts along the zone of NW-SE trending Neoarchean suture between the Dharwar and Bastar cratonic nuclei. The Proterozoic succession in the Valley comprises several major unconformity-bound sequences, each with a distinctive set of litho-assemblage, deposited under diverse tectonic environments with highly variable modes and tempos of sedimentation. Analysis of sedimentary attributes of unconformity-bound sequences indicates that Purana sedimentation in the Valley started with the deposition of stable platformal assemblage of carbonate-quartzarenite in an extensional sag basin. The basin opened at c. 1700 Ma. The stability of the craton was disrupted when the first major cratonic rift nucleated at c. 1620 Ma, and shortly evolved into an oceanic basin along the northeastern margin of the Valley with consanguineous deposition of a thick wedge of conglomerates, tidal sandstones and shales in the Mulug shelf, and a succession of graywacke, carbonate, siltstone-mudstone-black shale and tuff turbidites of the Somanpalli flysch in two parallel adjoining belts. The Somanpalli flysch is bounded by two shallow marine successions, analogous to several geosynclinal deposits preserved in orogenic belts, and was deposited in a deep oceanic basin along a rifted continental margin. Paleocurrent analysis reveals that siliciclastics in the flysch succession were derived primarily from a continental provenance to the southwest of the Mulug shelf, and were transported to the flysch basin across the margin. Coarse sands were deposited in large submarine fan complexes of graywackes and mixed carbonate-siliciclastics at several points. The voluminous carbonate turbidites in the flysch were also derived from the adjoining Mulug shelf. High quartz content of graywackes, their close association with tidally deposited arenites, and ocean-ward transport of sediments from cratonic hinterlands collectively attest to the deposition of the flysch in a trailing margin basin. Opening of the oceanic basin and deposition of the flysch were accompanied by extensive granulite facies metamorphism at deep crustal level indicating a genetic linkage between them, and the litho-tectonic elements were deformed into an orogenic belt during re-assembly of Dharwar and Bastar blocks. The orogenic belt girdled the northeastern margin of the PG Valley, wrapped around its southeastern corner making a syntaxial bend, and continued into the Ongole-NKSB orogenic belt which also had its peak metamorphism at c. 1600 Ma. The age of the orogeny is not well constrained, but was followed by a Neoproterozoic episode of extension, and formation of cratonic rift basin with the deposition of several thousand meters of quartz-rich sandstones of the Sullavai, Albaka and Usur sequences.

New SHRIMP U–Pb zircon ages from the Hartswater Group, South Africa: Implications for correlations of the Neoarchean Ventersdorp Supergroup on the Kaapvaal craton and with the Fortescue Group on the Pilbara craton

The Neoarchean Hartswater Group of the western Kaapvaal craton is a bimodal volcanic and sedimentary cratonic cover succession traditionally correlated with the Platberg Group of the ∼2.71Ga Ventersdorp Supergroup, South Africa. Correlation between exposures of the Platberg Group equivalents across the Kaapvaal craton is complicated, because they were deposited within isolated grabens, they display lateral facies changes over short distances, and they are extensively covered by calcrete and sand. Such correlation is important, since these units constitute one of the oldest unconformity-bounded sequences originally compared with sequences from the Pilbara craton (northwestern Australia) for reconstructing the ancient continent “Vaalbara”. Present age constraints, however, imply a ∼50 million year discrepancy in the shared geological histories of the cratons. Here we report SHRIMP U–Pb zircon ages from a prominent pyroclastic surge and ash fall deposit in the lower Hartswater Group (2733.4±3.4Ma) and from variable quartz–feldspar porphyry in the upper Hartswater Group (2724.3±5.8Ma). The new constraints significantly improve correlations of the Platberg Group equivalents on the western Kaapvaal craton, and present a clear solution to the apparent enigma in cross-craton correlation. The data cast doubt on the 2714Ma age for the Klipriviersberg Group of the east central Kaapvaal craton, and strengthen lithostratigraphic correlations with units from the Pilbara craton (i.e., the Hardey Sandstone, the Bamboo Creek and Spinaway porphyries, the Kylena Basalt, and in part the Tumbiana Formation). When our ages are placed within paleogeographic context a systematic picture of a shared long-lived extensional event emerges.

Palaeoproterozoic sedimentation in the Cuddapah Basin, south India and regional tectonics: a review

Abstract The Proterozoic Cuddapah Basin of south India hosts a number of unconformity-bound sequences deposited over continental crust. The Palaeoproterozoic Papaghani Group with basal polymict conglomerate unconformably overlies the Archaean Peninsular Gneiss and set the stage for Palaeoproterozoic sedimentation following a long hiatus. Two cycles of fluvial–shallow-marine psammite–carbonate–pelite sedimentation in the western part of the basin (Papaghni sub-basin) is punctuated at different levels by mafic flows, sills and dykes, and less common acidic tuffs. Each of the sedimentation cycles fosters extensive carbonate platforms with dolomitic stromatolites and algal laminites. Emplacement of shallow crustal intrusives at the end of the first cycle was possibly linked to thermal anomalies in the mantle and associated widespread crustal extension in south India. Brittle fault-slip analysis from the western Cuddapah indicates repetitive extensional to strike-slip regimes. This paper proposes a likely late Palaeoproterozoic shallow-marine sedimentary sequence of the Nallamalai Fold Belt as allochthonous, thrusted over the successions of the Papaghni sub-basin and the Kurnool Group in the western part of the basin. Inversion of the Papaghni sub-basin and the development of regional erosional unconformities is discussed in the context of Palaeoproterozoic and early Mesoproterozoic orogenic events at the SE margin of India.

Episodic Growth of the Chocolate Mountains Anticlinorium Recorded By the Neogene Bear Canyon Conglomerate, Southeastern California, U.S.A

Abstract Between Indian Pass and Picacho State Recreation Area, southeastern California, the Neogene Bear Canyon conglomerate crops out in and around the Chocolate Mountains anticlinorium, and can be subdivided into three unconformity-bounded sequences that are designated from oldest to youngest I, II, and III. Each of these sequences is composed of interstratified debris flow, channelized-stream-flow, and sheet flood deposits. Debris flow deposits are poorly sorted inversely graded pebble to boulder paraconglomerates with subangular clast populations. Large clasts extruding into overlying layers are a characteristic of such beds. In contrast, sheet flood and channelized-stream-flow deposits are well sorted orthoconglomerates with subrounded to rounded clasts. The Bear Canyon conglomerate unconformably overlies a lower Miocene volcanic and epiclastic unit, metasedimentary and metavolcanic rocks of the Jurassic Winterhaven Formation, a probable Jurassic orthogneiss, and the Upper Cretaceous to Paleogene Orocopia Schist. Geochemical, paleocurrent, and clast-count data suggest that alluvial material in the Bear Canyon conglomerate was derived from local exposures underlain by one or more of these units, and then was funneled into an approximately north–south-trending structural depression in the axial trace of the Chocolate Mountains anticlinorium. Though the exact timing of deposition of the three sequences in the Bear Canyon conglomerate is difficult to constrain, stratigraphic and published geochronological data, when viewed in the context of regional tectonic patterns, suggest that sequence I probably was deposited after ~ 17 Ma and before ~ 9.45 Ma. In contrast, sequence II was deposited after sequence I and then was folded either sometime prior to or after ~ 9.45 Ma. Sequence III is interstratified with the ~ 9.45 Ma basalts of Black Mountain, and was folded prior to deposition of flat-lying Quaternary alluvial sediments. The east–west-trending Copper basin reverse fault cuts sequence III. In addition, key components of a conjugate set of northwest- and northeast-trending dextral- and sinistral-slip faults, respectively, transect the Copper basin fault, the axis of the Chocolate Mountains anticlinorium, and the ~ 9.45 Ma basalts of Black Mountain. However, neither the Copper basin nor the conjugate faults disrupt flat-lying Quaternary alluvial sediments. Episodic growth of the east–west-trending Chocolate Mountains anticlinorium during deposition of the Neogene Bear Canyon conglomerate, and subsequent formation of the east–west-trending Copper basin fault and the conjugate set of dextral and sinistral faults all record transient north–south shortening within the margin of the North American plate. These transient periods of north–south shortening followed extensional deformation of lower Miocene continental-arc volcanic rocks, and are strain compatible with the ~ 12–10 Ma to ~ 6 Ma development of the eastern California shear zone. Numerous other examples of Late Cenozoic east–west-trending folds and reverse faults are reported in the literature to have formed during development of this 100–110 km wide zone of dextral shear. We, therefore, conclude that the geology and development of the Chocolate Mountains anticlinorium between Indian Pass and Picacho State Recreation Area reflect changing continental-margin strain patterns as the boundary between the Farallon and North American plates was transformed from a Paleogene convergent margin to a wholly Neogene transform setting.

Stratigraphic architecture of the fluvio-lacustrine deposits, Bathan Formation, Al-Rehaili area, North Jeddah, Saudi Arabia

A siliciclastic-dominated succession (∼11 m thick) underlying Harrat Rahat, belonging to the Miocene–Pliocene Bathan Formation is recently exposed at Al-Rehaili area, North Jeddah, Saudi Arabia. It covers a wide spectrum of grain sizes varying from clay-rich mudstones to cobble grade conglomerate and consists of a variety of facies vary from fluvial to marginal and open lacustrine deposited in a half-graben basin formed along the eastern margin of the extensional Red Sea Basin. Field-based sedimentologic investigation enables to identify ten facies grouped into three facies associations (A–C). The depositional history is subdivided into two stages. The first stage represents deposition in gravel to sand-dominated fluvial system sourced from a southern source and grade northward into lacustrine delta and open lacustrine setting. The second stage on the other hand includes deposition of fluvial channels running in E–W direction with attached bank sand bar. Sequence stratigraphic interpretations of the lacustrine deposits enable to identify three unconformity-bounded sequences (SQ1–3). The basal sequence is incomplete, consisting of three aggradationally to progradationally stacked delta plain and delta front parasequences. The second sequence is sharply and erosively overlying a red paleosol bed that defines the upper boundary of the first sequence. It includes two system tracts; upward-fining and deepening lacustrine offshore mudstones of the transgressive system tracts unconformably overlain by red paleosol of the regressive systems tracts. The top of this sequence is delineated at the sharp transgressive surface of erosion at the base of delta mouth bar deposits of sequence 3. Changes in the accommodation and sedimentation rates by basin subsidence under the influence of tectonics and sediment compaction and loading as well as climatic oscillation between semi-arid to arid conditions were the major controls on the fluvio-lacustrine sedimentation and their facies distribution. Tectonic reorganization of the drainage system resulted in the formation of E–W flowing fluvial streams in the second stage.

Hydrogeology, sequence stratigraphy and diagenesis in the Paleoproterozoic western Thelon Basin: Influences on unconformity-related uranium mineralization

The Paleoproterozoic (Statherian) Thelon Formation consists of unmetamorphosed sandstone and conglomerate that was deposited in the Thelon Basin, a continental basin in northern Canada. This study presents the first detailed sedimentological and sequence stratigraphic analysis of the expansive and under-studied western Thelon Basin, and its relation to uranium mineralizing processes in the vicinity of the Boomerang Lake unconformity-related U prospect. Four lithofacies comprise the Thelon Formation in the western Thelon Basin, consisting of breccia, conglomerate, sandstone, and minor mudstone. These are grouped into four facies associations that represent deposition in alluvial fans and braided fluvial streams. Three stratigraphic sequences were identified based on the recognition of widespread surfaces that mark significant periods of subaerial exposure or changes in depositional environment. These unconformity-bounded sequences are composite in nature and can be subdivided into six high-frequency sequences (HFS). Low-accommodation and high-accommodation systems tracts are delineated, instead of the traditional systems tracts that are linked to changes in relative sea level, as no marine deposits are recognized in the Thelon Formation. Two significant strataform paleohydrogeological units were distinguished in the Thelon Formation, based on the relative timing of key diagenetic events such as compaction, grain dissolution, and cementation. Diagenetic aquicludes became isolated from basinal fluid flow due to stylolite-catalyzed early diagenetic quartz cementation, and occlusion of secondary intergranular macroporosity by kaolinite and muscovite, and minor dickite during later peak diagenesis. Diagenetic aquifers conducted U-bearing basinal fluids throughout diagenesis and are marked by an abundance of peak diagenetic dickite. Sequence stratigraphic correlation indicates that diagenetic aquicludes, which were not effective at conducting U-bearing basinal fluids, overlie the unconformity at the Boomerang Lake prospect and extend along the entire strike length of a related exploration trend. In contrast, thick diagenetic aquifers lie approximately 10 km to the north of the Boomerang Lake prospect along a different exploration trend. The western and eastern portions of the Thelon Basin experienced depositional environments and diagenetic conditions distinct from one another, suggesting that they were separate sub-basins. The western Thelon Basin contains fewer of the well-cemented facies that compartmentalized basinal fluid flow than the eastern Thelon Basin, and contains a kaolin-dominated diagenetic mineral assemblage rather than a muscovite-dominated assemblage that is common in the eastern Thelon Basin. In these regards, the western Thelon Basin evolved more similarly to the eastern portion of the Athabasca Basin, where thick, regional, kaolin-rich diagenetic aquifers were fluid conduits for the movement of the U-rich fluids that formed high-grade unconformity-related U deposits.

Condensation origin for Neoproterozoic cap carbonates during deglaciation

Neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates. This stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions. In contrast to this, sequence stratigraphic field studies of an Ediacaran (ca. 635 Ma) glacial to postglacial transition in the Amadeus Basin of central Australia reveal a complex deglacial stratigraphy, in which more than 175 m of conglomerate, sandstone, marl, and carbonate at the basin margin, and portions of four unconformity-bounded sequences, pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies. The unconformities, which are characterized by as much as several tens of meters of erosional relief (oblique sections of incised valleys), separate intervals of contrasting sediment provenance, and are confidently mapped on the basis of both criteria. Comparable unconformities are absent in the overlying Neoproterozoic succession, which is >2 km thick and encompasses many tens of millions of years. The Amadeus Basin cap carbonate was thus deposited during a protracted interval of multiphase (cyclical) transgression more similar to Phanerozoic cyclical sea-level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate. The superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions. Facies lithologically similar to cap carbonates may be less obvious in Phanerozoic successions because of a secular change in carbonate composition to reefal and deep-sea pelagic deposits.

Controls on active forearc basin stratigraphy and sediment fluxes: The Pleistocene of Hawke Bay, New Zealand

Detailed, high-resolution documentation of forearc basin fi ll is scarce in the literature. In this geological and geophysical study, we investigated the Pleistocene sedimentary record of the tectonically active Hawke Bay forearc domain of the Hikurangi subduction margin of New Zealand. Interpretation of an extensive seismic-refl ection data set that is correlated with marine cores and onshore geological maps identifi es the detailed stratigraphic architecture of the last ~1.1 m.y. This analysis reveals the infl uences and interactions of tectonic deformation, climate, eustasy, and isostasy on forearc basin sedimentation. Eleven ~100 k.y. depositional sequences are recognized in the basin fi ll, thus highlighting the dominance of Pleistocene climate-eustasy on sequence development. The stacking pattern and isopach maps of sequences exhibit an overall retrogradational trend and an arcward migration of depocenters. These trends progressively develop a basinwide diachronous and composite erosion unconformity formed by the lateral succession and landward encroachment of the 12 sequence-bounding unconformities (S12 to S1). Among these, the S5 surface (ca. 430 ka) is an angular unconformity that separates major megasequences of the sedimentary record. The forearc domain evolved from a series of ridge-parallel basins to a succession of connected basins that have progressively developed around major, growing thrust-faulted ridges since ca. 430 ka. This change in basin confi guration and associated signifi cant increase of the preserved sediment fluxes occurred synchronous with the reactivation of major out-of-sequence thrusts and the completion of the mid-Pleistocene transition.

Active synsedimentary tectonism on a mixed carbonate-siliciclastic continental margin: third-order sequence stratigraphy of a ramp to basin transition, lower Sekwi Formation, Selwyn Basin, Northwest Territories, Canada

The lower part of the Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, is composed of two regional, unconformity-bounded sequences, S0 and S1, which record the first widespread carbonate deposition during the initial Palaeozoic transgression onto the western margin of Laurentia. These Early Cambrian sequences are unique to the western North American Cordillera, representing the only record of primarily deep-water deposition on a tectonically active, mixed carbonate–siliciclastic ramp during this period. More specifically, the geometry of the Sekwi ramp changed during deposition of S0 and S1, from a shallowly dipping homoclinal ramp during the S0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of S0. The steeply dipping ramp profile of S0 was preserved into the early transgressive systems tract of S1. The Sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of S1 and remained so throughout the remainder of Sekwi deposition. The evolving shape of the Sekwi ramp is attributed to syndepositional ‘down to the basin’ faulting during deposition of both S0 and S1 and is recorded by: (i) the westward thickening, irregular geometries of S0 and S1; (ii) geographical restriction of deep-water facies (including sediment gravity flow deposits); (iii) the presence of large allochthonous blocks; and (iv) the clast composition of sediment gravity flow deposits. Sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower Sekwi Formation, as they delineate depositional packages, including the maximum flooding zone, the transitions between portions of systems tracts, and the inferred locations of syntectonic extensional faults. Syntectonic faults increased accommodation basinward of an extensive ooid-shoal complex that developed along the Sekwi ramp crest, greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows. The syndepositional faulting probably was a continuation of extension that began during the latest Neoproterozoic rifting of western Laurentia. The composition of sediment gravity flow deposits track changing accommodation space on the lower Sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy.

Sequence evolution in the eastern Chhattisgarh Basin: constraints on correlation and stratigraphic analysis

The Proterozoic succession in the eastern part of the Mesoproterozoic Chhattisgarh Basin comprises two unconformity-bounded sequences. Sequence I represents the Chhattisgarh Supergroup of earlier workers. It overlies rocks of the basement complex with a profound unconformity. Sequence II unconformably overlies Sequnce I, and represents the closing phase of basin evolution during the early Neoproterozoic time. It is unconformably overlain by rocks of the Gondwana Supergroup.
 The Lohardi and Gomarda formations at the lower part of the Chandarpur Group of Sequence I comprise an immature succession of conglomerate, sandstone and shale deposited in fan-delta - pro-delta environments, marked by rapid facies changes, variable rates of sediment influx, and uneven rates of subsidence and creation of accommodation space. The Kansapathar Sandstone in the upper part of the Chandarpur Group, by contrast, comprises a sheet of mature arenite deposited in a macrotidal shelf. The immature assemblage is best developed in the eastern part of the basin, and rapidly thins out towards west, where the Kansapathar arenite directly overlies the basement. The Raipur Group provides an excellent example of cyclic sedimentation of red shale and limestone. It comprises three shale-dominated intervals and two carbonate-dominated intervals, organized into multiple shallowing-up cycles. The lower carbonate succession, the Sarangarh Limestone, developed as a shallow water un-rimmed platform and evolved into a deep water ramp, with an extensive thin sheet of black limestone facies. Stromatolites are conspicuously absent in the Sarangarh Limestone. Small stromatolite bioherms appear in the Gunderdehi Shale which overlies the ramp succession, and abundant growth of stromatolite is noted in the upper carbonate succession which evolved as a rimmed platform. A thick ignimbrite horizon in the Churtela Shale attests to major felsic volcanism and termination of the Sequence at ~1000 Ma.
 The Kansapathar Sandstone, the black limestone facies of the Sarangarh Limestone, and the Gunderdehi Shale embedded with small stromatolite bioherms can be used as key marker horizons to overcome the problem of intrabasinal correlation. The marker horizons can be traced from the western part to the eastern part of the basin. The stromatolites in the Gunderdehi Shale and in the Saradih Limestone further provide a biostratigraphic frame, subject to detailed morphologic and microstructural analysis, for possible chronostratigraphic classification.

K-bentonite, black-shale and flysch successions at the Ordovician–Silurian transition, South China: Possible sedimentary responses to the accretion of Cathaysia to the Yangtze Block and its implications for the evolution of Gondwana

The K-bentonite, black shale and flysch successions at the Ordovician–Silurian transition in South China have been the subject of comprehensive investigations relative to the probable accretion of the Yangtze Block and the questionable Cathaysia Block. First, the geochemical analyses of K-bentonites show that the parent magma originated in syn-collisional, volcanic-arc and within-plate tectonic settings, which produced mainly intermediate-to-felsic series magmas, associated with continuous collision and subduction of paleo-continental blocks/arcs. Further, the regional distribution of K-bentonite thickness indicates that voluminous explosive volcanism was located in the present southeastern shoreline provinces of China. Secondly, northwestwardly migrating, Ordovician–Silurian, transitional flysch successions, and the accompanying diachronous K-bentonite-bearing black-shale interval, as well as the related, overlying, shallowing-upward succession at the interior of the Yangtze Block, developed as an unconformity-bound sequence that mirrors foreland-basin tectophase cycles in the Appalachian basin. The above features suggest that the sequence accumulated in a similar foreland basin, which formed in response to adjacent deformational loading in a northwesterly migrating orogen located to the southeast. Geochemical and paleocurrent data from the turbiditic flyschoid sandstones also support these depositional settings. Accordingly, it seems that all criteria strongly support the presence of an Ordovician–Silurian, subduction-related orogen resulting from collision with a block to the southeast that must have been the original “Cathaysia Block” of Grabau and later workers. The K-bentonite, black-shale and flysch successions can be regarded as distal, foreland responses to the continuous northwestward collision and accretion of the Cathaysia Block to the Yangtze Block. Hence, we prefer to suggest that the suture zone with the sensu stricto Cathaysia Block probably developed along previously identified late Early Paleozoic suture relicts in the shoreline provinces of southeast China. On the other hand, although accretion of fragments with Cathaysian affinities to the Yangtze Block may have begun as early as Middle to Late Proterozoic time, the Ordovician–Silurian orogeny described above probably reflects the final phase of accretion between the two blocks. Moreover, when combined with similar peri-Iapetan orogenic events in other areas during the same period, this accretion event may have been part of a major stage of global tectonic reconstruction in the evolution of Gondwana.

Carson Pass-Kirkwood paleocanyon system: Paleogeography of the ancestral Cascades arc and implications for landscape evolution of the Sierra Nevada (California)

Tertiary strata of the central Sierra Nevada are dominated by widespread, voluminous volcanic breccias that are largely undivided and undated, the origin of which is poorly understood. These dominantly andesitic strata are interpreted to be eruptive products of the ancestral Cascades arc, deposited and preserved within paleocanyons that crossed the present-day Sierra Nevada before Basin and Range faulting began there. These strata are thus important not only for understanding the paleogeography of the Ancestral Cascades arc, but also for reconstructing the evolution of the Sierra Nevada landscape. Our regional-scale mapping shows that paleocanyon fills of the central Sierra Nevada are dominated by intrusions and vent-proximal facies along the present-day Sierran crest, with more distal facies extending westward down the paleocanyons. Vent-proximal facies consist of lava domes that collapsed to generate block and ash flow tuffs, which in turn were remobilized down-canyon to produce coarse-grained volcanic mudflow and dilute flow (fluvial) deposits. Lava flows are rare; instead, magmas invaded wet volcaniclastic sediment to form in situ peperite piles that were partly remobilized to form debris flows or block and ash flows with peperite domains. Detailed mapping and dating of previously undifferentiated Tertiary strata in the Carson Pass–Kirkwood area of the central Sierra Nevada has allowed us to identify 6 unconformity-bounded sequences preserved within a paleocanyon as deep as 650 m cut into Mesozoic granitic basement. The Carson Pass–Kirkwood paleocanyon trends NE-SW, with a paleo-transport direction roughly parallel to the modern Mokelumne River drainage (toward the SW). Sequence 1 consists of Oligocene silicic ignimbrites sourced from Nevada. Sequences 2 through 6 consist of dominantly andesitic rocks of the Miocene Ancestral Cascades arc, with 40 Ar/ 39 Ar ages ranging from 14.69 ± 0.06 Ma to 6.05 ± 0.12 Ma. These new dates provide constraints on the ages of unconformities. Vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters; with paleoslope gradients range from 3° to 48°. No evidence exists for widening or deepening of the paleocanyon into Mesozoic basement during Tertiary time, so we infer that unconformity 1 (the paleocanyon floor and walls) was inherited from Cretaceous time. The deepest unconformities within the paleocanyon fill reincised into granitic basement in the early Miocene (unconformity 2), between 14 and 10 Ma (unconformity 5), and between 10 and 6 Ma (unconformity 6). The paleocanyon was also beheaded (cut off from sources to the east) by ca. 10 Ma. We suggest that the early Miocene reincision records tectonism related to the onset of arc magmatism in the Sierra Nevada. The middle Miocene reincision may record the onset of Basin and Range faulting in the central Sierra. The late Miocene reincision may correspond to uplift attendant with the northward sweep of the triple junction through the latitude of the central Sierra. Paleocanyons of the central Sierra differ from those of the northern and southern Sierra by showing steeper local paleorelief, and the bouldery stream deposits attest to higher axial gradients than envisioned for other parts of the range. This may indicate that the uplift history of the range is not uniform from segment to segment.

High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture

Research Article| July 01, 2007 High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture Anthony C. Runkel; Anthony C. Runkel 1Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114-1057, USA Search for other works by this author on: GSW Google Scholar James F. Miller; James F. Miller 2Department of Geography, Geology and Planning, Southwest Missouri State University, Springfield, Missouri 65804, USA Search for other works by this author on: GSW Google Scholar Robert M. McKay; Robert M. McKay 3Iowa Department of Natural Resources, Geological Survey Bureau, 109 Trowbridge Hall, Iowa City, Iowa 52242-1319, USA Search for other works by this author on: GSW Google Scholar Allison R. Palmer; Allison R. Palmer 4Institute for Cambrian Studies, 445 North Cedarbrook Road, Boulder, Colorado 80304, USA Search for other works by this author on: GSW Google Scholar John F. Taylor John F. Taylor 5Geoscience Department, Indiana University of Pennsylvania, 129 Weyandt Hall, Indiana, Pennsylvania 15705, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (7-8): 860–881. https://doi.org/10.1130/B26117.1 Article history received: 07 Oct 2006 rev-recd: 27 Feb 2007 accepted: 19 Mar 2007 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anthony C. Runkel, James F. Miller, Robert M. McKay, Allison R. Palmer, John F. Taylor; High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture. GSA Bulletin 2007;; 119 (7-8): 860–881. doi: https://doi.org/10.1130/B26117.1 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 Well-known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental-scale tectonism, oceanographic dynamics of epeiric seas, and eustasy. We used a multi-disciplinary approach to construct a high-resolution stratigraphic framework for lower Paleozoic strata in the cratonic interior of North America. Within this framework, these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins, settings markedly different from the cratonic interior. Parasequences, parasequence stacking patterns, systems tracts, maximum flooding intervals, and sequence-bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification. The similarity of cratonic interior and foreland basin successions in size, geometry, constituent facies, and local stacking patterns of nearshore parasequences is especially striking. This similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same, despite marked differences in tectonism, physiography, and bathymetry between the two settings.Our results do not support the widespread perception that Paleozoic cratonic interior successions are so anomalous in stratal geometries, and constitute such a poor record of time, that they are poorly suited for modern sequence stratigraphic analyses. The particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another. Thus, Paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography. Special conditions of exceptionally slow subsidence rate, shallow bathymetry, and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings. These results suggest that if other cratonic interior successions are as anomalous as reported, a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture. The results also lead us to suggest that a nonvegetated lower Paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones, particularly their widespread yet thin geometry, and a scarcity of shale and siltstone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

SEA-LEVEL CHANGE AND THE STRUCTURE OF MARINE ECOSYSTEMS

The emergence of sequence stratigraphy—the study of genetically related facies within a framework of chronostratigraphically significant surfaces—triggered a conceptual breakthrough in stratigraphy (Van Wagoner et al., 1990; Baum and Vail, 1998) and vastly improved our understanding of the fossil record (Holland, 1999). Sequence-bounding unconformities and transgressive surfaces are the record of relative sea-level changes and a signature of time intervals not represented in the geological record. This is a key issue for those interested in documenting the steps in the history of life. Despite a good sampling of the rock record (Donovan and Paul, 1998), concern that the sedimentary record is biased remains widely justified by recent studies (Smith, 2001; Crampton et al., 2003). Rock biases and sedimentary gaps strongly determine what we perceive as abundance and diversity patterns (Raup, 1972; Smith, 2001; Peters, 2005). Sequence stratigraphic architectures, however, also allow the role of sea-level changes to be tested as a driving force of ecology and evolution in the marine realm. Sea-level variations as sources of minor and major environmental perturbations (Hallam and Wignall, 1999) are factors in ecosystem structure (Olszewski and Erwin, 2004) and macroevolution (Peters, 2006). A session of the 2004 American Association of Petroleum Geologists Dallas Meeting entitled, “Sea-level change, climate variation, and the restructuring of shallow marine communities,” addressed paleobiological factors related to this important aspect of sequence stratigraphy. Some participants of that session and other researchers have contributed their thoughts on the subject in this special issue of PALAIOS. Before sequence stratigraphic concepts made their way into paleontologic practice, authors studying the biotic effects of relative sea-level change used formations as the natural subdivisions of the rock record and derived sea-level templates from 106–107-year duration transgressive-regressive cycles (e.g., Rollins et al., 1979; Jablonski, 1980; Hallam, …

Early Cambrian acritarch assemblage zones in South Australia and global correlation

Lower Cambrian sediments in the Arrowie and Stansbury Basins were deposited on a rifting continental platform and contain abundant acritarchs, trace fossils, small shelly fossils, archaeocyaths, trilobites and brachiopods. Generally, faunas and acritarchs are abundant in transgressive and highstand deposits, but less common in lowstand fluvial deposits, responding both to sedimentary environments and preservational conditions. Seven acritarch Assemblage Zones are recognised in the lowest Cambrian to Toyonian sediments of South Australia. Assemblage Zone 1 embraces the Precambrian-Cambrian boundary, but a dramatic increase of spinose acritarchs (Assemblage Zone 2) is observed around 1 m above the first appearance of the trace fossil Rusophycus. Assemblage Zone 3 occurs in transgressive siltstones during deposition of the uppermost Parachilna Formation and is marked by the first appearance of several taxa, which are widespread and well correlated in South Australia. Assemblage Zones 4, 5 and 6 are dominated by Skiagia; the appearance of S. ornata followed by S. ciliosa in Zones 4 and 5, respectively, indicates correlation with the nominal acritarch zones on the East European Platform and concurrent Schmidtiellus mickwitzi and Holmia kjerulfi trilobite zones, and the Qiongzhusi Formation in southern China. Assemblage Zone 7 encompasses a major acritarch decrease in South Australia during the late early Cambrian regression and 'red-beds' deposition. Continuous occurrences of acritarch genera Skiagia, Ceratophyton and Corollasphaeridium during the early Cambrian provide valuable evidence of evolutionary development, and therefore are reliable for biostratigraphic zonation. The integration of biostratigraphy and unconformity-bounded sequence stratigraphy provides more precise intercontinental correlation.