Lowermost Sequence Research Articles

Paleozoic Paraná Basin transition from collisional retro-foreland to pericratonic syneclise: Implications on the geodynamic model of Gondwana proto-Andean margin

Providing significant advancements in the knowledge and development of the global plate tectonic theory, the Paraná Basin records a long period of sedimentary accumulation with evidence of substantial climatic and bio-stratigraphic changes throughout most all of the Phanerozoic. Two cycles of sedimentation are identified in the lowermost sequences (Rio Ivaí and Paraná groups), followed by a ~50 Ma gap until the deposition of the early Gondwanic cycle (Itararé Group). The literature suggests that some of those depositional cycles were influenced by episodes of terrane accretion during the amalgamation of the Gondwana Supercontinent. The paleo-topography generated by the Ocloyic orogeny (460-430 Ma) likely resulted in flexural subsidence in the lowermost stratigraphic cycles of the Paraná Basin. In response to dynamic topography, the forebulge probably shifted to a system dominated by long-wavelength propagation developed over a distance of more than 800 km, most likely due to the influence of the subducted oceanic lithosphere of Panthalassa. The presence of Ordovician sources in the pre-Carboniferous sequences supports a provenance of exhumed Famatinian magmatic arc and associated rocks given that Ordovician magmatic activity is not recorded anywhere else in the cratonic regions. Furthermore, important detrital zircon population yielding Cambrian ages, likely derived from Eastern Pampean Range. In additional, the gap between crystallization and depositional ages within the Paraná Basin supports a model that transitions from a collisional retro-foreland basin to a pericratonic syneclise after the deposition of the Paraná Group.

Earliest Campanian - latest Maastrichtian sequence stratigraphy based on planktonic foraminifera, Fars province, Zagros, Iran

The Gurpi, Tarbur and Sachun formations have been investigated in the studied section in the Fars Province, in order to determine their sequence stratigraphy. On the basis of done studies on the cores of borehole, four main microfacies have been recognized in four stratigraphic sequence deposited during the Campanian to Maastrichtian. The lowermost sequence, was deposited in the early Campanian-early late Campanian which include wackestone to packestone texture with Globotruncanita elevata Zoneand Globotruncana ventricosa Zonethatrepresentative of the deep marine and outer ramp. The intermediate and uppermost sequences (sequences two, three and four) display well developed deposits formed in the end of the Campanian (Radotruncana calcarata Zoneand Globotruncanella havanensis Zone), formed during the Maastrichtian (Globotruncana aegyptiaca Zoneand Gansserina gansseri Zone) and the end of the Maastrichtian (Omphalocyclus macroporus - loftusia sp assemblage Zone) on pelagic, hemipelagic, outer and middle ramp. On the basis of the sequence stratigraphic chart, the transgression of the upper Cretaceous sea started since the early Campanian and continued gradually until the early Maastrichtian. Then, until the end of Maastrichtian, the area has been emerged. The sequence stratigraphic architecture of Campanian/ Maastrichtian Gurpi, Tarbur and Sachun formations model is in a good agreement with global sea level changes.

Detrital zircon provenance and palaeogeographic implications of the Ediacaran Shigu Group in the Zhongza Terrane, SW China

ABSTRACT The palaeogeographic positions of South China in the Rodinia and Gondwana supercontinents have long been controversial. We report petrological, detrital zircon U-Pb geochronological, trace element geochemical and Lu-Hf isotopic compositions for metasedimentary rocks from the Shigu Group in the Zhongza Terrane, SW China. The Shigu Group has been regarded as the Precambrian basement of the Zhongza Terrane, and comprises mainly a suite of medium-/low-grade metamorphosed terrigenous clastic rocks. The youngest zircon age population of two metasedimentary rock samples from the bottom Shigu Group yields a weighted average age of 633 ± 9 Ma, indicative of a late Neoproterozoic (ca. 630 Ma) maximum deposition age. The youngest detrital zircon age of a metasedimentary rock from the upper Shigu Group is 597 ± 7 Ma, it may suggesting a considerably later deposition. Detrital zircon age distribution patterns of the Shigu Group metasedimentary rocks are similar to those of coeval sedimentary rocks in the western Yangtze Block. According to Hf isotope compositions and zircon morphology, the Neoproterozoic Shigu Group detritus may have sourced from Neoproterozoic magmatic rocks in the western Yangtze Block, whilst the pre-Neoproterozoic zircons might originate from recycling of the underlying metasedimentary rocks. Our results indicate that the basement of the Zhongza Terrane was a continuation of the Yangtze Block, which implies that Precambrian Yangtze basement rocks were present beyond the current boundary of the Block. In the late Neoproterozoic (ca. 630–570 Ma), the western Yangtze Block may have been facing the Proto-Tethys and received detritus mainly from the local Neoproterozoic magmatic rocks. During the late Neoproterozoic-earliest Palaeozoic (ca. 570–520 Ma), the seaway closure between the Yangtze (incl. Zhongza) Block and northern India likely created new pathways for delivery of the earliest Neoproterozoic and Pan-African detrital zircons into the lowermost Palaeozoic sequences in South China.

The influence of mechanically weak layers in controlling fault kinematics and graben configurations: Examples from analog experiments and the Norwegian continental margin

Fault systems in extensional basins commonly display geometries that vary with depth, reflecting depth- and lithology-dependent mechanical strength. Using an experimental approach, we investigate this relationship by deploying physical analog models with stratified sequences consisting of brittle–ductile (sand–silicone polymer) sequences subject to single and polyphase deformation. The experiments were used as analogs for a sandstone sequence interlayered by beds of evaporates or overpressured or unconsolidated mudstone in nature (the latter being representative of decollement horizons). Experiments (series 1 [S1]) using homogeneous and stratified quartz and feldspar sand produced asymmetric, composite single grabens with diverse fault frequencies and fault styles for the graben margin faults. For the mechanically stratified experiments with one decollement level (series 2), contrasting graben configurations were produced, in that the lowermost sequence was characterized by graben geometries of similar type to that of the S1 experiments, whereas the sequence above the decollement was characterized by large fault blocks, delineated by steepened or oversteepened faults. The experiments with two decollements (series 3) were displayed similarly but included graben geometries that widened upward, with each level being characterized by independent fault systems. The results can be used to explain strata-bound fault patterns and depth-dependent extension as seen in several places along the Norwegian continental margin and elsewhere.

Geological inheritance and its role in the geomorphological and sedimentological evolution of bedrock-hosted incised valleys, lake St Lucia, South Africa

The morphology of Lake St Lucia, Africa's largest estuary, has been strongly influenced by antecedent topography throughout its Quaternary evolution. The 5 × 20 km southern basin is bounded to landward by a 50 m-high Early/Mid-Pleistocene dune ridge and to seaward by a 100 m-high Late Pleistocene/Holocene dune barrier. Nine seismic units (A-I) are identified in the enclosed basin, along with two regionally developed sequence boundaries (SB1 and SB2) and two tidal ravinement surfaces (tRS1 and tRS2). The lowermost sequence boundary (SB1) is related to regional late-Pliocene hinterland uplift. SB2 is related to the last glacial maximum (LGM) when sea levels fell ∼130 m below present. Lowstand fluvial sediments occupy the base of both SB1 and SB2 incisions. Thick central basin deposits dominate the valley fills, more so in the Pliocene age valleys, the occurrence and thickness of which is attributed to sheltering by a seaward barrier-dune system as well as to the low gradient settings. Prograding tidal bedforms occupy the minor incisions formed within the tidal ravinement surfaces, marking the onset of lagoonal conditions in the area. The system is capped by lagoonal deposits, deposited above tidal flats. A series of prograding spits mark the early stages of lagoonal segmentation in the Holocene. The preservation of only one incision and fill sequence prior to the development of an LGM-aged valley succession is attributed to geological inheritance whereby older valleys were repeatedly re-incised by younger valleys throughout the Pleistocene, causing partial preservation of Pleistocene fills up to the most recent cycle. The older fill remnant relates to a larger magnitude base level fall prior to the Quaternary, which survived erosion during subsequent sea-level falls. Adjacent topography in the form of rocky cliffs and the seaward fronting barrier limit the accommodation space in which infilling material can be hosted outside the major valley network. The competent basement rock into which these valleys have incised protected lowstand fluvial sediments from subsequent tidal ravinement.

Biostratigraphy and Sequence Stratigraphy of Campanian and Maastrichtian Succession, Qutbabad Well, Zagros (Iran)

The Qutbabad section of the Gurpi, Tarbur and Sachun Formations in the Fars Province (Zagros Basin) has been studied, in order to determinate its microfacies and sequence stratigraphy. On the basis of thin section studies of cores from Qutbabad Well in the Gurpi, Tarbur and Sachun Formations of the Zagros Basin, four main microfacies have been recognized within four stratigraphic sequences deposited during the Campanian to Maastrichtian. Depth and sea level changes were the main controls on the distribution of these microfacies. The microfacies are: Pelagic; Hemi-Pelagic; Outer ramp; and Middle ramp. The lowermost sequence was deposited in the Early Campanian-early Late Campanian and includes wackestone to packestone textures with biozones Globotruncanita elevate and Globotruncana ventricosa representative of the Pelagic, Hemipelagic and Outer (ramp/shelf). The intermediate and uppermost sequences (sequence two, three and four) display well developed deposits formed at the end of the Campanian (Radotruncana calcarata zone and Globotruncanella havanensis zone), beginning of the Maastrichtian (Globotruncana aegyptiaca zone and Gansserina gansseri zone) and the end of the Maastrichtian (Omphalocyclus macroporus - loftusia sp assemblage zone) on the Pelagic, Hemipelagic, Outer and Middle ramp. On the basis of the sequence stratigraphic chart, the transgression of the Upper Cretaceous Sea started from the Early Campanian and continued gradually towards the Early Maastrichtian. Then, until the end of Maastrichtian, has had regression. This resulted that the sequence stratigraphic model of the Campanian- Maastrichtian Gurpi, Tarbur and Sachun Formations has architecture similar to those that have developed from global sea level changes.

A loess deposit in the Late Triassic of southern Gondwana, and its significance to global paleoclimate

This work aims to characterize the depositional systems of the Santa Maria Supersequence (Middle to Late Triassic) in the Paraná Basin and discuss their paleoclimatic significance through the use of field, chemical and paleontologic data. Facies analysis led to the identification of two dominant facies associations: dry mudflat and sheet delta. The dry mudflat facies association dominates in the Pinheiros-Chiniquá and Santa Cruz Sequences, and sheetflood delta facies association occurs only in the Candelária Sequence. Loess deposits were recognized in the lowermost sequences. Evidence for loess deposits includes the absence of floodplain feeding channels, thick, structureless siltstone bodies, overwhelming dominance of grain-size mode in 0.0031 mm and age dispersion of dated zircons. Throughout the Santa Maria Supersequence, the chemical index of alteration indicates semi-arid climate, in agreement with global-scale paleoclimatic conditions. However, during the Carnian the abundance of calcrete decreased and the occurrence of aquatic and semi-aquatic fauna increased, implying an increase in humidity in this interval. Other fossiliferous basins in Gondwana recorded the same short-lived, humid event. The humid peak may be associated with the Carnian “Wet Intemezzo”, related with the development of the Wrangellian large igneous province (LIP).

Sequence stratigraphy, basin morphology and sea-level history for the Permian Kapp Starostin Formation of Svalbard, Norway

AbstractBased on seven measured sections from Svalbard, the marine strata of the Permian Kapp Starostin Formation are arranged into seven transgressive–regressive sequences (TR1–TR7) ofc. 4–5 Ma average duration, each bound by a maximum regressive surface. Facies, including heterozoan-dominated limestones, spiculitic cherts, sandstones, siltstones and shales, record deposition within inner, middle and outer shelf areas. The lowermost sequence, TR1, comprises most of the basal Vøringen Member, which records a transgression across the Gipshuken Formation following a hiatus of unknown duration. Temperate to cold, storm-dominated facies established in inner to middle shelf areas between the latest Artinskian and Kungurian. Prolonged deepening during sequences TR2 and TR3 was succeeded by a long-term shallowing-upward trend that lasted until the latest Permian (TR4–TR7). A major depocentre existed in central and western Spitsbergen while to the north, Dickson Land remained a shallow platform, leading to a shallow homoclinal ramp in NE Spitsbergen and Nordaustlandet. The Middle Permian extinction (late Capitanian) is recorded near the base of TR6 in deeper parts of the basin only; elsewhere this sequence is not recorded. Likewise the youngest sequence, TR7, extending to the upper formational contact of latest Permian age, is found only in the basin depocentre. Comparison with age-equivalent strata in the Sverdrup Basin of Canada reveals a remarkably similar depositional history, with, for example, two (third-order) sea-level cycles recorded in the Late Permian of both regions, in keeping with the global record. Sequence stratigraphy may therefore be a powerful correlative tool for onshore and offshore Permian deposits across NW Pangaea.

Regional T-R sequence stratigraphy and lithostratigraphy of the Bearpaw Formation (Upper Campanian), west-central and southwestern Alberta plains

Abstract In west-central and southwestern Alberta, the lower and upper tongues of the marine mudstone-dominated Bearpaw Formation have a complex lateral relationship with marginal marine and non-marine strata of the Horseshoe Canyon and Blood Reserve formations. Wireline log, core and outcrop data permit the establishment of a regional transgressive-regressive (T-R) sequence stratigraphic framework for the upper Campanian Bearpaw strata. This provides a context within which lithostratigraphic boundaries of the Bearpaw tongues with laterally equivalent and overlying strata can be more rigorously mapped. Maximum flooding (MFS) and transgressive (TS) surfaces were defined on the basis of stratal stacking patterns and inferred lapout relationships wherever possible. Lower Bearpaw tongue and laterally equivalent strata are assigned to four regionally mappable T-R sequences. In the study area the top of the underlying Belly River Group is placed at the MFS in the lowermost of these sequences. In the overlying lower Bearpaw sequences, the TS is generally considered to coincide with the MFS within the limits of wireline log resolution, and the succession consists mainly of stacked regressive systems tracts which usually show a near-parallel internal stratal geometry. Upper Bearpaw tongue and laterally equivalent strata are assigned to three regionally mappable T-R sequences. The basal upper Bearpaw sequence has a well-developed transgressive systems tract consisting of backstepping, individually regressive parasequences, and southeast-prograding clinoform geometry is clear in each of the upper Bearpaw tongue regressive systems tracts. The lithostratigraphic upper boundaries of the lower and upper Bearpaw tongues are time-transgressive facies contacts that step up-section to the southeast (paleoseaward) through the respective regressive systems tracts. The base of the upper Bearpaw tongue steps up-section from the basal TS of the lowermost sequence in the southeast, through the overlying transgressive systems tract, to coincide with the maximum flooding surface to the northwest (paleolandward).

Compound incised-valley characterization by high-resolution seismics in a wave-dominated setting: Example of the Aude and Orb rivers, Languedoc inner shelf, Gulf of Lion, France

A branch of a time-calibrated late Quaternary compound incised valley complex is investigated using high- and very-high-resolution seismic data. The incised valley system is confined on the inner shelf, and entrenched parallel to the shore in unconsolidated Pliocene deposits. The infilling of the incised valley system comprises three sigmoidal bodies dipping progressively downstream representing depositional sequences. The lowermost sequences are less well preserved at their downstream extremity and the whole system is both aggrading and prograding. Older Pleistocene/Late Quaternary sequences could be preserved under the coastal plain. Individual sequences are closely similar to the classic model of a microtidal incised valley fill. Nevertheless, the central estuary/bay basin muds are seen to interfinger locally with high-energy deposits that represent potential reservoirs. The properties (prograding and aggrading architecture and occurrence of high energy deposits) and preservation of these compound incised valley fill deposits are attributed to general (glacio-eustatic cycles) and local (atmospheric and oceanic regime and proximity of the hinge line) conditions. Data acquisition strategy is a determining factor to interpret such systems.

Sequence Architecture of the Bighorn Dolomite, Wyoming, USA: Transition To the Late Ordovician Icehouse

Abstract Although major Late Ordovician glaciation was confined to the terminal Ordovician Hirnantian Stage, lasting less than 2 million years, recent isotopic evidence suggests a prolonged interval of slow growth in polar ice caps. Tropical carbonate platforms from passive margins and epicontinental seas offer the opportunity to monitor changes in eustatic sea-level changes that accompanied this transition from a greenhouse towards an icehouse climate. The Late Ordovician Bighorn Dolomite of northern Wyoming and adjacent states preserves such a transition. The Bighorn Dolomite was deposited on a northeastward-facing, subequatorial platform, and it consists of three third-order depositional sequences. Each is bounded by a regional erosional surface, commonly with erosional truncation visible at outcrop scale and commonly mantled by a distinctive brecciated horizon. The Bighorn Dolomite is dominated by highly bioturbated and fossiliferous open marine shallow subtidal facies, but portions also contain dolomitic bioturbated restricted marine shallow subtidal and laminated peritidal facies, as well as thin-bedded argillaceous and fossiliferous deep subtidal limestone. The three depositional sequences preserve a transition in their internal cyclicity that is consistent with computer modeling of the stratigraphic signal of increasing period and amplitude of eustatic sea-level changes. The lowermost sequence, which spans the Lander and Steamboat Point members, consists of homogeneous, acyclic, shallow subtidal facies, with a few meter-scale peritidal parasequences near its top. The middle sequence, which corresponds to the Leigh Member, contains acyclic to weakly cyclic restricted shallow subtidal and peritidal facies. The uppermost sequence, consisting of the Horseshoe Mountain Member, is dominated by five decameter-scale parasequences composed of open marine shallow subtidal, restricted shallow subtidal, and peritidal facies. Overall, the Bighorn Dolomite is interpreted to represent deposition during a shift from low-amplitude (a few meters), short-period (~ 20 kyr) cyclicity to moderate-amplitude (~ 10 m), long-period (100 kyr) cyclicity indicating a shift from greenhouse conditions towards icehouse conditions, a shift that agrees with recent isotopic evidence for changing Late Ordovician climate.

Birth and demise of a Middle Jurassic isolated shallow-marine carbonate platform on a tilted fault block: Example from the Southern Iberian continental palaeomargin

Subbetic Middle Jurassic oolitic limestones of the Jabalcuz Formation crop out in San Cristóbal hill, near Jaén city (Andalucía, Spain), between hemipelagic limestone and marl successions. The Jabalcuz limestones range in facies from calcareous breccias and micritic limestones to white cross-bedded oolitic limestones. Recent erosion has exhumed a Jurassic isolated shallow-water carbonate platform on the San Cristóbal hill. This shallow platform developed on a tilted fault block. An almost continuous, laterally extensive outcrop reveals tectono-sedimentary features distinctive of block-tilting in the different margins of the fault block. The studied sections represent various palaeogeographic positions in the ancient shallow-water carbonate platform and basin transition. This exceptional outcrop allows to decipher the triggering mechanisms of the birth, evolution, and drowning of this Jurassic isolated shallow-water carbonate platform. Two shallowing-upward depositional sequences separated by flooding surfaces can be distinguished on two different sides of the fault block. In the southeastern part of the outcrop, proximal sections grade vertically from distal talus fault breccias, with bivalve and serpulid buildup intercalations, to white cross-bedded oolitic limestones defining the lowermost depositional sequence. Upwards, overlying a flooding surface, the second sequence with oolitic limestones prograding over micritic deposits is recorded. In the southwest, oolitic, peloidal, and more distal micritic facies alternate, with notable southeastern progradation of oolitic facies in the upper part of the section, which represents the upper depositional sequence. The top of this second depositional sequence is another flooding surface recorded by the sedimentation of marls with radiolarians from the overlying formation.In the northwestern outcrops, the two depositional sequences are also almost completely preserved and can be differentiated. A 100m-thick succession composed of a basal slumped unit of micritic and peloidal limestones overlain by cross-bedded oolitic limestones represents the lowermost sequence. The superposition of a newer slumped unit of micritic and peloidal limestones on the cross-bedded oolitic limestones defines the lower part of the uppermost depositional sequence. Finally, the central sections are less thick and comprise a facies succession consisting almost entirely of oolitic limestones.Locally, oolitic limestones often show cross-bedding and prograding clinoforms. The cross-bedding has centrifugal directions around the perimeter outcrops of San Cristóbal hill, whereas in the centre palaeocurrents are highly variable. The sense of the palaeoslopes as deduced from slumps also fits with a centrifugal pattern. Finally, in the southwestern outcrops, the demise of the carbonate platform is marked by backstepping oolitic facies, the presence of lumachelle facies, and onlapping hemipelagic facies.All the data point to the birth, evolution, and drowning of an isolated shallow-water carbonate platform that developed on a tilted fault block. The fault-block tilting controlled the development of talus breccias in the southeastern part and of slumps in the northwest-dipping hangingwall ramp. In the shallowest environments produced by the tilting (uppermost part of the fault block), a shallow-marine carbonate platform developed. Carbonate production (mainly oolitic) kept up with relative sea-level fluctuations and extended the platform at least to the northwest, southeast, and southwest based on observable data. Finally, tectonics and sea-level fluctuations drowned this carbonate platform.

Microfacies and sequence stratigraphy of the Amapá Formation, Late Paleocene to Early Eocene, Foz do Amazonas Basin, Brazil

On the basis of thin-section studies of cuttings and a core from two wells in the Amapá Formation of the Foz do Amazonas Basin, five main microfacies have been recognized within three stratigraphic sequences deposited during the Late Paleocene to Early Eocene. The facies are: 1) Ranikothalia grainstone to packstone facies; 2) ooidal grainstone to packstone facies; 3) larger foraminiferal and red algal grainstone to packstone facies; 4) Amphistegina and Helicostegina packstone facies; and 5) green algal and small benthic foraminiferal grainstone to packstone facies, divisible locally into a green algal and the miliolid foraminiferal subfacies and a green algal and small rotaliine foraminiferal subfacies. The lowermost sequence (S1) was deposited in the Late Paleocene–Early Eocene (biozone LF1, equivalent to P3–P6?) and includes rudaceous grainstones and packstones with large specimens of Ranikothalia bermudezi representative of the mid- and inner ramp. The intermediate and uppermost sequences (S2 and S3) display well-developed lowstand deposits formed at the end of the Late Paleocene (upper biozone LF1) and beginning of the Early Eocene (biozone LF2) on the inner ramp (larger foraminiferal and red algal grainstone to packstone facies), in lagoons (green algal and small benthic foraminiferal facies) and as shoals (ooidal facies) or banks ( Amphistegina and Helicostegina facies). Depth and oceanic influence were the main controls on the distribution of these microfacies. Stratal stacking patterns evident within these sequences may well have been related to sea level changes postulated for the Late Paleocene and Early Eocene. During this time, the Amapá Formation was dominated by cyclic sedimentation on a gently sloping ramp. Environmental and ecological stress brought about by sea level change at the end of the biozone LF1 led to the extinction of the larger foraminifera ( Ranikothalia bermudezi).

Upper Vendian-lowest Ordovician sequences of the western Gondwana margin, NE Spain

Abstract The intra-Vendian (Ediacaran)–intra-Tremadocian succession of the Cadenas Ibéricas in NE Spain is divided into nine sequences. Overall, these are interpreted as second-order sequences. Those that encompass Lower and lower Middle Cambrian carbonates, with identified transgressive systems tract (TST) and highstand systems tract (HST) phases, may constitute elements of composite sequences. The lowermost sequence is of Late Vendian age. In Lower and lower Middle Cambrian units, sequence tops indicate drowning, reflecting extensional tectonics. Rifting effects are traceable up to mid-Mid Cambrian times. The remaining sequences probably represent a sag phase, either accentuating the preceding extensional local basin regime or heralding the Gondwana passive margin stage. Of the southerly Gondwana deposits those of other areas of the Iberian Peninsula and of the Moroccan Anti-Atlas fold belt show similar conditions during the Early Cambrian, that is, an Early Cambrian extensional regime, and, as for the Moroccan fold belt, four sequences imaging TST and HST phases.

The stratigraphy of the Neogene-Quaternary succession in the southwest Netherlands from the Schelphoek borehole (42G4-11/42G0022) – a sequence-stratigraphic approach

AbstractWe investigate the stratigraphy of Neogene and Quaternary intervals of the Schelphoek borehole (Schouwen, Zeeland, the Netherlands). The Breda Formation (Miocene-Zanclean) contains three sequences separated by hiatuses. The Oosterhout Formation (Zanclean-Piacenzian) contains at least two sequences. This formation is overlain by seven sequences of the Gelasian Maassluis Formation that almost certainly represent glacial cycles. The three lowermost sequences are provisionally assigned to the Praetiglian (MIS 96, MIS 98 and MIS 100). A large hiatus exists between the top of the Maassluis Formation and the base of the late Middle to Late Quaternary succession. Due to extensivein situreworking of older strata (including fossils) at the base of several of the formations, their exact boundaries are difficult to establish. The Neogene succession in the Schelphoek borehole is compared to the stratigraphic successions in the Antwerp area to the south and the Dutch coastal area and continental platform to the north. Finally, the stratigraphic context of the Gelasian (‘Tiglian’) mammal fauna dredged from the bottom of a major tidal channel in the adjacent Oosterschelde is assessed by comparison with the Schelphoek borehole.

Late Quaternary evolution of macrotidal Kimpo tidal flat, Kyonggi Bay, west coast of Korea

Up to 32 m thick late Quaternary deposits are developed in the Kimpo tidal flat, Kyonggi Bay, west coast of Korea. The deposits consist of six lithofacies units that are grouped into three unconformity-bounded sequences, which formed in response to late Quaternary sea-level fluctuations. The lowermost sequence (S3) consists of late Pleistocene fluvial deposits (Unit VI) and overlying tidal deposits (Unit V). Unit VI is interpreted to have formed in a meandering channel and associated floodplain environment. Weak pedoturbation characterizing the upper part of Unit VI suggests that pedogenesis occurred prior to initial transgression. Well-developed tidal rhythmicities in Unit V indicate that tidal sedimentation prevailed during the first marine inundation. Intermediate sequence (S2) is composed of late Pleistocene fluvial deposits (Unit IV) and overlying tidal deposits (Unit III). Depositional environments of Unit IV and Unit III are similar to those of Unit VI and Unit V, respectively. Upper parts of Unit IV and Unit III are variably indurated and pedoturbated, suggesting prolonged subaerial exposure. The uppermost sequence (S1) consists of early Holocene sphaerosiderite-rich marsh deposits (Unit II) and overlying middle to late Holocene tidal deposits (Unit I). Pervasive occurrence of similar tidal rhythmites in both Holocene and late Pleistocene tidal deposits indicates that tidal environment recurred over several sea-level cycles. Age dates and stratigraphic position indicate that Holocene tidal deposits were formed since 7–8 ka BP. Given the validity of age dates, however, timing of late Pleistocene tidal deposits is controversial in the light of eustatic sea-level curves. Considering tectonically stable condition of study area, the elevation of the late Pleistocene tidal deposits is too high to fit the curves. Elucidation of the exact timing of the tidal deposits requires more independent age controls as well as further understanding on the tectonic regime of the west coast of Korea.

Ages of Detrital Zircon from Siliciclastic Successions South of the São Francisco Craton, Brazil: Implications for the Evolution of Proterozoic Basins

In this work we report 207Pb/ 206Pb LA-ICPMS ages of 152 detrital zircons from lower greenschist facies quartzites from Proterozoic basin successions of the southern border of the São Francisco Craton, southern Minas Gerais State, Brazil. These are the intracratonic São João del Rei basin, the intraplate continental margin Andrelândia basin, and the Serra do Ouro Grosso sequence, developed on a crystalline basement older than 1.8 Ga, and deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59–0.50 Ga. The data constrain both the ages of the sources and the interval of sedimentation. The detrital zircons of the Serra do Ouro Grosso sequence were derived predominantly from the erosion of a Neoarchean crust, 2.5–2.8 Ga old, with only one grain showing a Paleoproterozoic age (2, 245±83 Ma) older than the Transamazonian event. Zircons extracted from a shelf quartzite of the lowermost sequence of the São João del Rei basin indicate derivation from the 1.8–2.2 Ga Transamazonian crust, with subordinate contribution from the 2.5–2.9 Ga Archean crust. The 1, 809±41 Ma age is interpreted as the maximum limit for sedimentation in this basin. The results confirm the regional correlation with the Espinhaço Rift successions. The zircons extracted from an autochthonous quartzite of the Andrelândia sequence yielded ages in the 1.0–2.2 Ga range, with a modal class at 1.2–1.3 Ga. Only two of the forty analyzed zircons yield Archean ages. The youngest zircon yields 1, 086±85 Ma. The zircons from the allochthonous quartzite yield ages between 1.0–2.7 Ga, with a modal class at 2.1–2.2 Ga. Only five of 45 analyzed grains yield Archean ages. The youngest zircon has an age of 1, 047±77 Ma. The results indicate that the detrital sediments deposited during the second marine flooding event of the Andrelândia sedimentation were mainly derived from the erosion of Mesoproterozoic and Paleoproterozic rocks. The 1, 047±77 Ma age is interpreted as the maximum depositional age for the described association.

SEQÜÊNCIAS E TRATOS DE SISTEMAS DEPOSICIONAIS DO GRUPO ITARARÉ, NORTE DO ESTADO DO PARANÁ

A representative vertical stratigraphic section about 800m thick was measured in outcrops of the Carboniferous-Permian Itarare Group in northern Parana State. Five depositional sequences bounded by disconformities were recognized and their facies associations described. The outcrop section shows good correction with well logs and the sequences may be traceable over 400 kilometers across the basin in a EW depositional strike section. The five sequences have the same basic stratigraphic signature, which represents the glaciomarine sedimentary response to advance and retreat of glaciers. Ideally, three successive facies associations would compose each sequence, but the lower and upper ones may be absent. The lower facies association occurs only in the two lowermost sequences and includes subglacial facies as tillites and boulder conglomerates (glacial maximum system tract). The subglacial facies are covered by conglomerates and sandstones, which in turn are overlain by diamictites, turbidites and finely laminated facies with dropstones, composing a typical retrogradational facies stacking resulting from deglaciation (deglaciation system tract). Lying directly over the disconformities when subglacial facies do not occur, the deglaciation facies association is present in all sequences and represents the bulk of the stratigraphic record of the Itarare Group. Fine-grained laminated facies represent the record of the maximum glacial retreat during interglacial periods and contain the maximum seawater-flooding surface of each sequence. Coarsening upward facies association may be present in the upper part of the sequences, interpreted as the sedimentary response to the beginning of new glacial advances (glacial advance system tract). This facies association could represent progradation in highstand system tracts, but coastal and continental facies were not observed in the studied area.

Petrographic provenance analysis of Kiowa Core sandstone samples, Denver Basin, Colorado

To assess provenance variations during Late Cretaceous–Tertiary uplift of the Colorado Front Range, detailed petrographic analysis was performed on core samples from the Kiowa Cored Well, Elbert County, Colorado. Forty-two samples from five stratigraphic intervals were used in the analysis. Samples range in depth from 91.5 to 2,242 ft (27.9 to 683.4 m). Results indicate that minor but significant amounts of low-grade, metasedimentary debris occur in the Fox Hills Sandstone and Pierre Shale samples and require a relatively distal, westerly source. Subsequent to a strong volcanic pulse during deposition of the Upper Cretaceous Laramie Formation and lowermost D1 sequence, a gradual unroofing sequence is recorded in the overlying Upper Cretaceous–middle Paleocene D1 synorogenic deposits. The volcanics encountered in the Laramie and lowermost D1 sequence are primarily silicic types. An influx of chert detritus to sandstones of the lower D1 sequence is interpreted to reflect unroofing of post-Lyons Sandstone, chertrich, Mesozoic sandstone. This was followed by overlapping depositional pulses of feldspathic and then chert- and dolomiterich sandstone. The first of these is interpreted to have coincided with unroofing of the arkosic Fountain Formation and the second with the removal of the underlying lower Paleozoic carbonates and clastics. The shallowest occurrence of components having a definite sedimentary origin is at a depth of 976.2 ft (297.5 m) in the upper D1 sequence. The shallowest sample containing quartz that exhibits significant rounding, indicating derivation from sedimentary sources, occurs at a depth of 1,061 ft (323.4 m). Abundances of feldspathic components exhibit a dramatic increase in the upper D1 sequence and continue into the lower Eocene D2 sequence sandstones, where they comprise the bulk of the framework minerals. A second major pulse of volcanic debris occurs in the uppermost D1 sequence. The D2 sequence was derived almost exclusively from plutonic materials, with no evidence indicating input from high-grade metamorphics (feldspathic gneiss and amphibolite) that tend to dominate the Front Range foothills north of the Castle Rock area.

Geochronologic evidence for Early Cretaceous volcanic activity on Barton Peninsula, King George Island, Antarctica

Ages of six volcanic and plutonic rocks on Barton Peninsula, King George Island, were determined using 40Ar/39Ar and K-Ar isotopic systems. The 40Ar/39Ar and K-Ar ages of basaltic andesite and diorite range from 48 My to 74 My and systematically decrease toward the upper stratigraphic section. Two specimens of basaltic andesite which occur in the lowermost sequence of the peninsula, however, apparently define two distinct plateau ages of 52-53 My and 119-120 My. The latter is interpreted to represent the primary cooling age of basaltic andesite, whereas the former is interpreted as the thermally-reset age caused by the intrusion of Tertiary granitic pluton. The isochron ages calculated from the isotope correlation diagram corroborate our interpretation based on the apparent plateau ages. It is therefore likely that volcanism was active during the Early Cretaceous on Barton Peninsula. When the K-Ar ages of previous studies are taken into account with our result, the ages of basaltic andesite in the northern part of the Barton Peninsula are significantly older than those in the southern part. Across the north-west-south-east trending Barton fault bounding the two parts, there are significant differences in geochronologic and geologic aspects.