Progradational Sequence Research Articles

Quaternary alluvial fan sedimentation in the Dehradun Valley Piggyback Basin, NW Himalaya: tectonic and palaeoclimatic implications

ABSTRACT The Dehradun Valley, a synclinal intermontane valley piggyback basin within the Siwalik Group rocks in the NW Himalaya, is separated from the Lesser Himalayan formations in the north by a major intraplate thrust, the Main Boundary Thrust (MBT) and from the Indogangetic Plains in the south by the Himalayan Frontal Fault (HFF). Major parts of the Dehradun Valley are covered by three fans, from west to east the Donga, Dehradun and Bhogpur fans, deposited by streams following the topography produced by activity of the MBT and probable footwall imbricate thrusts, starting at about 50 ka.The Donga and Dehradun fans were fed by small streams and characterized mainly by sediment gravity‐flow deposits (debris flow and mudflow deposits) in the proximal zone, and mostly mudflow deposits and minor braided stream deposits in the middle zone during the period 50–10 ka. Palaeosols were weakly developed in the proximal zone and moderately to strongly developed in the middle zone. The degree of development of palaeosol was mainly a function of rate of sedimentation and to some extent entrenchment of streams into the fan surface. Since 10 ka, deposition has been typically by braided streams. The Bhogpur fan has been marked by deposition from relatively larger braided streams since 50 ka.The fan sequences in the Dehradun Valley are synorogenic and their deposition started due to activity of the southern footwall imbricate of the MBT, i.e. Bhauwala Thrust on the Donga and Dehradun fans. In these fans, major fan sequences show retrogradation (50–10 ka) related to a decrease in the activity of the MBT and related imbricates and activity of more hinterlandward imbricates with time. After 10 ka a thin prograding sequence was deposited due to uplift of the fans, which resulted from the activity on a thrust in the distal parts of the fans. It suggests an out‐of‐sequence activity of faults in the MBT imbricate system. Cross‐faults divide the Siwalik formations in the footwall of the MBT into three blocks, which were marked by decreasing subsidence or possibly uplift from east to west. Thrusting on the HFF was not piggyback type but synchronous with activity of the MBT and its imbricates. The development of the Mohand fault‐bend anticline above the HFF changed the nature of the basin from foreland to piggyback type, shed minor colluvial deposits prior to 10 ka, and folded the southernmost fan deposits in the western, narrow parts of the valley.A major change in climate from a cold, dry climate with strong seasonal variations prevailing since 50 ka to warm and humid climate at about 10 ka resulted in a change in depositional processes from sediment gravity‐flows to braided streams.

Comparison of Evolution of Trioctahedral Chlorite/Berthierine/Smectite in Coeval Metabasites and Metapelites from Diagenetic to Epizonal Grades

AbstractThe evolution of texture, structure and chemical composition of chloritic clays in coeval pairs of metabasites and metapelites of a prograde sequence from the Bükk Mountains has been investigated using electron microscopy techniques. Samples are from the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine metamorphism, ranging from late diagenesis to epizone for pelites and from prehnite-pumpellyite to greenschist facies for the metabasites.Although bulk-rock compositions, textures and primary minerals are different, chlorite evolved at similar rates in coeval metabasites and metasediments, but along different paths. The principal similarities in the prograde sequence are a decrease in the percentage of interstratified material in both dioctahedral and trioctahedral phyllosilicates and increase in thicknesses of chlorite and illite crystallites. The principal difference is in the type of interstratification in chlorite, with berthierine in metapelites, and smectite (saponite) in metabasites, although smectitic mixed layers also occur in the former. The evolution of trioctahedral phyllosilicates is marked by a decrease in the number of mineral species with increasing grade, chlorite, sensu stricto, being the only trioctahedral mineral at higher grades. This is consistent with the trend in reaction progress where both metastable systems (metabasites and metapelites) tend toward the same end-member, thermodynamically stable chlorite, as well as texture (crystal size), and where all intermediate states are metastable, and determined by the Ostwald step rule.

Foreland-basin sequence response to collisional tectonism

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

Qasim Formation: Ordovician Storm- and Tide-Dominated Shallow-Marine Siliciclastic Sequences, Central Saudi Arabia

ABSTRACT The Middle to Upper Ordovician Qasim Formation is well exposed in the Qasim region of central Saudi Arabia and is recognized from many wells to the north and east. It consists of the Hanadir, Kahfah, Ra’an, and Quwarah members that are arranged in two coarsening-upward progradational sequences that have a total thickness of about 200 m in outcrops. Two sections were measured in each sequence. The lower sequence is a storm-dominated, shallow-marine depositional system composed of the late Llanvirn to middle Caradoc Hanadir and Kahfah. At least five progradational beach parasequences were identified. The Hanadir overlies deltaic deposits of the upper Saq Formation and marks a major marine transgression onto Gondwana. It is composed of laminated fissile shale formed in an offshore marine environment together with siltstone laminae that indicate periodic influxes of low-density turbidity currents. The overlying Kahfah is composed of shale and sandstone. The upper progradational sequence consists of the middle Caradoc to middle Ashgill Ra’an and Quwarah members. The Ra’an is a succession of homogeneous fissile shale and minor siltstone laminae formed in an offshore marine environment following a major transgression onto the Kahfah. The overlying Quwarah consists of sandstone and minor amounts of siltstone and shale in its lower parts, and a thickly bedded sandstone facies above. The diagnostic sedimentary structures in the Quwarah are large-scale lens-shaped tidal sand waves composed of sigmoidal bundles deposited in a mesotidal setting of barrier bars, tidal channels, ebb-dominated deltas and lagoons. In the subsurface, the Formation thickens northeastward (basinward) to over 4,000 ft (1,220 m). In the same direction, the sand-dominated Kahfah and Quwarah gradually become shale-dominated, small-scale bed forms are more abundant, and identification of members based on lithofacies is more difficult. After deposition of the Quwarah, present-day Saudi Arabia was affected by a Late Ordovician (late Ashgill) glaciation. The Qasim and older formations are deeply incised by glacial channels at the base of the Zarqa and Sarah formations. Basinward, the glacial unconformities (or their submarine erosion surface equivalents) become less significant and a thick succession of the Formation is preserved.

The sequence stratigraphic framework of carbonate diagenesis within transgressive fan-delta deposits: Sant Llorenç del Munt fan-delta complex, SE Ebro Basin, NE Spain

Abstract The Eocene (Bartonian–Priabonian) Sant Llorenc del Munt fan-delta complex formed at the front of the Catalan Coastal Range as a result of Alpine deformation. Alluvial fan and fan-delta progradation into the Ebro Basin was arrested by repeated transgressive episodes, thus producing a hierarchy of transgressive–regressive sequences. This study focuses on the two transgressive–regressive fundamental sequences that comprise the El Marcet Unit, a succession of predominantly clastic deposits of shallow-marine and coastal plain origin. Sandstones and conglomerates within the El Marcet Unit are cemented by calcite. Under cathodoluminescence these cements display varying degrees of luminosity. On the basis of differences in luminosity and stable isotopic composition, temporal relationships between the various cements have been established. Cementation commenced in the soil zone with the local formation of calcretes. Early burial cements precipitated in the vadose zone from waters of meteoric origin, and later cements in the shallow phreatic zone. Stable isotopic evidence suggests that all cements probably precipitated from pore waters of essentially the same origin, but at varying burial depths and with varying contents of organically derived carbon. Calcite cements formed by the remobilisation of biogenic and clastic carbonate, and from pedogenic CO 2 . The formation of early cements was facies-dependent and principally controlled by the distribution of shell material. Variations in cement distribution related to the observed cyclicity of the unit are also apparent. The abundance of early cements increases towards the top of progradational sequences, and early cements dominate the upper sequence, while late cements dominate the lower sequence.

Correlating Pleistocene sequences across the New Jersey margin

Abstract The biostratigraphic ranges of palynomorphs, as well as climatostratigraphic changes recorded by both marine and terrestrial palynomorphs, provide one of the few means of dating and correlating neritic environments where calcareous microfossils are rare. The transport of terrestrial palynomorphs to marine environments also allows for direct land–sea correlation. In addition, palynological assemblages provide insights into sediment transport and sea levels. Over 500 m of upper Pleistocene sediments were recovered in ODP Hole 1073A on the upper New Jersey slope. Palynomorphs in this Hole record rapid progradation during the late Pleistocene. A prominent unconformity, pp3(s), forms the upper boundary of this thick progradational sequence. Palynological analysis from ODP Sites 1072 and 1073 allows this surface to be correlated from the outer shelf to the upper slope, and also suggests that it was generated during a sea level lowstand, probably during Oxygen Isotope Stage 12 (∼450–425 ka). Both the Atlantic Coastal Plain and ODP Site 1072 on the outer New Jersey shelf appear to have become erosional during this progradational phase which began around the time that the magnitude of Northern Hemisphere ice accumulation increased. This suggests that sediments prograded across the New Jersey margin as accommodation in inner neritic and coastal environments decreased in response to amplified glacioeustatic fluctuation and substantially increased sediment availability (supplied by glacial erosion). Erosion associated with the generation of unconformity pp3(s) appears to have increased accommodation on the New Jersey margin, allowing sediments to aggrade on the continental shelf and even to accumulate on the Atlantic Coastal Plain during interglacial highstands. The upper Pleistocene architecture of the New Jersey margin appears to have been controlled by glacioeustatic fluctuations and the accompanying increase in sediment supply. In contrast, unconformable surface pp4(s) is more complex and separates upper Miocene sediments deposited between 7.4 and 5.9 Ma from lower Pleistocene sediments deposited prior to approximately 1.4 Ma in Hole 1072A, whereas in Hole 1073A, pp4(s) is in sediments of early Pleistocene age, between 1.6 and 1.3 Ma. These data suggest that unconformity pp4(s) was generated during the early Pleistocene, but there is no clear palynological evidence that glacioeustasy was responsible for the generation of this unconformity.

Sequence and isotope stratigraphy of Late Triassic distal periplatform limestones from the Northern Calcareous Alps (Kälberstein Quarry, Berchtesgaden Hallstatt Zone)

The Kalberstein quarry at Berchtesgaden exposes Carnian-Norian deeper-water Hallstatt limestones. Conodont biostratigraphy, microfacies and stable isotopes of bulk carbonate matrix were investigated. The biostratigraphic results demonstrate a complete succession from the latest Carnian (Tuvalian 3/I) to the late Norian (Sevatian 2). As expected from the periplatform setting of the Hallstatt Zone, calculated mass accumulation rates conform partly to prograding sequences observed on the contemporary Dachstein platform. However, discrepancies exist, mainly for the middle Norian, pointing to an incomplete knowledge of the platform sequences. The sequence stratigraphic framework based on platform data should be complemented with data from the periplatform Hallstatt Zone. Diagenetic alteration of the limestones from Kalberstein quarry is low with a conodont alteration index (CAI)=1.0 throughout the section. Oxygen isotope values ranging from ± 1.2 to + 0.1½ (VPDB) point to stabilization and cementation at very shallow burial depths in contact with seawater in a deeper-water environment. Carbon isotope values display a clear stratigraphic trend with a rapid increase from 3.6 to 4.1½ (VPDB) during the basal Norian (Lacian 1), high values up to 4.2½ during the Lacian 2, and a slow decline starting in Lacian 3 to 2.6½ at the end of the Norian (Sevatian 1±2). These trends are best explained by variations in the global organic carbon/carbonate burial ratio with maximum organic carbon burial during the middle Lacian.

Geochemistry of Precambrian and Paleozoic siliciclastic rocks from the Iberian Range (NE Spain): implications for source-area weathering, sorting, provenance, and tectonic setting.

The major and trace element chemical composition of Precambrian and Paleozoic shales and sandstones from the Iberian Range (Spain) has been investigated to determine the provenance and tectonic setting of these rocks, as well as to appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The samples studied belong to a prograde sequence from diagenesis to anchizonal grade, and are mainly composed of quartz, illite/mica phases and/or kaolinite, with chlorite, feldspars, and carbonates as minor components. The major element distribution (Chemical Index of Alteration (CIA) indices) reflects that recycling processes have been important in homogenizing the composition of shales and sandstones. The higher CIA indices observed in the Silurian and Devonian shales (80 and 85, respectively) compared with those of Precambrian to Ordovician and the Carboniferous age (about 70) indicate that their source area underwent more intense chemical weathering processes, possibly due to climatic and/or tectonic variations. Alternatively, the source area of the Silurian and Devonian shales may have been composed of recycled sedimentary materials. Mineral fractionation is mainly observed in coarser rocks through zircon, apatite and xenotime accumulation, although the shales are not free of these phases. The zircon content of the fine-grained rocks is not high enough to affect rare earth element (REE) contents, but phosphate minerals (apatite and xenotime) at least partially control the REE distribution. The influence of apatite and xenotime are slightly higher in the Precambrian shales as the phosphate concentration produces a decrease in the La/Sm ratios. Chondrite-normalized REE patterns and negative Eu anomaly size of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating that they originally come from a differentiated silicic source. The higher REE contents in the studied shales in relation to PAAS indicate that recycling processes in the Iberian Range sources were probably more intense than that of PAAS. The slight differences among the REE patterns of the different groups of shales probably do not reflect changes in source-area composition, but instead suggest variations in mineral sorting, chemical weathering and/or sediment recycling. The Th/Sc, Co/Th, Cr/Th, Cr/V, V/Ni ratios support a primitive silicic source for these rocks, with the higher Th/Sc and lower Co/Th in Cambrian and Ordovician shales indicating a higher proportion of felsic material in their primitive source area. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.

Evolution of the southern Taiwan–Sinzi Folded Zone and opening of the southern Okinawa trough

Recent interpretation of seismic sections and free-air gravity anomalies in offshore northern Taiwan reveals that the southern Taiwan–Sinzi Folded Zone began to form in late Middle Miocene, though it was mainly constructed in the Late Pliocene with strong reverse faulting and folding. Two westward progradational sequences were deposited in the shelf basin with sediments supplied from the southern Taiwan–Sinzi Folded Zone and the southern Ryukyu Arc. These two structures are displaced by several northwest-striking dextral strike–slip faults that were active in the early Quaternary when the clockwise-rotated southern Ryukyu Arc and the folded southern Taiwan–Sinzi Folded Zone were broken. It is believed that recent extension in the southern Okinawa Trough started in the early Quaternary because uplift on the southern Taiwan–Sinzi Folded Zone continued to latest Pliocene–early Quaternary. Paleogene–Miocene sediments of the East China Sea Shelf in the western part of the southern Okinawa Trough Basin are interpreted to indicate that the East China Sea Shelf Basin extended to the east of the southern Taiwan–Sinzi Folded Zone.

Thrust-top lacustrine–lagoonal basin development in accretionary wedges: late Messinian (Lago-Mare) episode in the central Apennines (Italy)

We analysed in detail the geometry, sedimentology and palaeontology (both mollusc and ostracod assemblages) of a small lacustrine–lagoonal basin located at about 1100 m a.s.l. in the Marsica region of the Apennines, at Le Vicenne locality (L'Aquila, Abruzzo, central Italy). The structure of the basin infill deposits outlines an open syncline with an E–W axis. Polygenic conglomerates interbedded with clay levels and sandy marls, which constitute roughly coarsening-upward progradational sequences, are the dominant lithologies. These sequences are interpreted as lacustrine to coastal lagoon deposits with episodic fan delta supplies. This sedimentological interpretation is also supported by mollusc and ostracod assemblages, which suggest that the sedimentary environment was characterized by a mesohaline closed water body, with salinity probably ranging from 10‰ to 13‰. Both mollusc and ostracod assemblages show a clear Paratethyan influence. In the ostracod assemblage ten species out of the fourteen collected forms belong to the Loxoconcha djaffarovi Zone as defined by Carbonnel (1978). Thus, it is possible to refer the siltitic–clayey intercalations of Le Vicenne to the top of the Messinian (post-salinity crisis). The presence of a strong angular unconformity at the base of the basin, together with the structural setting of its deposits, suggesting a light compressive deformation, are common features in all the thrust-top basins recognized in the central Apennines. Taking into account the regional data, the age of the youngest deposit involved in the substratum (siliciclastic turbidites which recorded the Messinian salinity crisis) and the palaeobiological indications given by the mollusc and ostracod assemblages, it is possible to constrain the origin of the Le Vicenne basin to the Messinian Lago-Mare–Early Pliocene tectono-sedimentary event of the Apennines. The recognition in the Marsica range of this thrust-top basin allows us to attempt a palinspastic reconstruction locating the Marsica region in the latest Miocene–Early Pliocene active thrust zone of the Apennine accretionary wedge.

“Retrograde Diagenesis” of Clay Minerals in the Precambrian Freda Sandstone, Wisconsin

AbstractSiltstones from the Precambrian Freda Formation in Wisconsin have been studied by scanning, scanning-transmission and analytical electron microscopy (SEM, STEM and AEM), and X-ray diffraction (XRD). XRD data for drill core samples show a change from smectite-rich Illite-Smectite (I-S) in shallow samples to illite in deeper samples, implying a transition during burial diagenesis.Transmission electron microscopy (TEM) observations of shallow samples reveal the presence of three clearly distinguishable kinds of dioctahedral clay minerals: (1) detrital grains of micrometer-sized, mature muscovite; (2) small packets in the matrix consisting of dominant (Reichweite) R1 I-S or (3) small packets of illite. The illite and I-S stacks occur separately and have similar textures, with packet thicknesses averaging ∼400 Å, within the range of anchizonal illite. Illite and detrital muscovite commonly display strain features typical of the effects of tectonic stress. Void space within I-S or illite packets is inferred to be a strain feature, and to have served as pathways for fluids. Detrital muscovite shows abundant alteration features including (001) boundaries which are continuous with parallel packets of I-S; individual layers commonly show along-layer transitions of muscovite to smectite or I-S. Trioctahedral clays consist primarily of detrital chlorite which commonly shows direct alteration to R1 I-S and smectite, as with detrital muscovite.Deep samples contain only unaltered, coarse detrital muscovite, and thin packets of illite forming stacks and comprising most of the matrix. The texture of the illite appears to be identical to that of shallow samples, with characteristics such as packet size typical of anchizonal illite. Trioctahedral clays consist almost entirely of detrital grains of chlorite and corrensite. They occur as separate grains with rather constant composition, without signs of alteration.The data imply that all of the studied rocks have been subjected to a uniform anchizonal grade of metamorphism in which detrital grains were largely unchanged but matrix clays were transformed to packets of illite. The unusually abrupt transition with depth from highly expandable I-S to illite is inferred to actually be the result of subsequent alteration of authigenic illite and detrital chlorite and muscovite to R1 I-S and smectite in shallow rocks. This late overprinting of the anchimetamorphic clay mineral assemblage is inferred to have been locally caused by fluids with temperatures less than those of peak metamorphism. This process, called “retrograde diagenesis”, gave rise to a sequence of dioctahedral I-S and illite which mimics classic prograde sequences. Interpretations of such sequences as being prograde, especially in cases of ancient rocks, should be interpreted with caution when high-resolution images of textures are not available.

Non-marine ostracod assemblages of the Pre-Salt rift basins of West Africa and their role in sequence stratigraphy

Abstract Early Cretaceous ostracods living within the rift lakes of Central Gondwana reflect, through their morphology, abundance and species diversity, depositional environments and changes in water chemistry that occurred within the lakes at that time. By studying the indigenous ostracod assemblages it is possible to interpret past environmental changes with respect to both water depth and water chemistry, and to utilize this information to estimate the probability of source and reservoir sediment deposition. The alternation of siliclastic and carbonate sequences throughout rift lacustrine sediments can be explained by changes in the depositional environments and related water-level movements. Such changes being brought about as a direct result of tectonism leading to increases or decreases in precipitation as a consequence of river capture or regional changes in climate. Two cycles of sedimentary infill are recognized, each initiated by an onlapping, fining-upwards transgressive systems tract within which a maximum flooding surface is developed. This is followed by a coarsening-upwards highstand systems tract in which a prograding sequence retains the diverse ostracod fauna of the transgressive systems tract below. A coarsening-upwards lowstand systems tract, having a low diversity ostracod fauna, brings each cycle to a close. An understanding of the cyclic sequence stratigraphy of these basins suggests that the break-up of Gondwana commenced as early as the late Barremian. Commercially, these rift basin lakes contain excellent hydrocarbon source rocks, but it is the interpretation of the lacustrine faunas that provides the essential information as to lake size and hydrocarbon potential.

A Predictive Model for Reservoir Distribution in the Permian (Leonardian) Clear Fork and Glorieta Formations, Robertson Field Area, West Texas

Through the collaborative efforts of a multidisciplinary team, various subsurface data types have been integrated to produce a conceptual geologic model for reservoir prediction within the upper Clear Fork and Glorieta formations of the Robertson field area, west Texas. Detailed description of 1434 m of core, 109 thin sections, and 241 line-kilometers of 2-D (two-dimensional) seismic indicates the stratigraphic interval accumulated as a progradational succession of platform-interior subtidal and intertidal facies. Reservoir intervals preferentially occur within subtidal facies having intercrystalline porosity associated with replacement by coarsely crystalline dolomite. Intertidal facies were replaced by a finely crystalline phase of fabric-preserving dolomite and are dominated by an ineffective fenestral pore system. Facies and reservoir distribution is largely controlled by antecedent topography. Structural highs generated during the terminal phase of Ouachita-Marathon compression were the preferred site of intertidal facies deposition. Adjacent structural lows have a higher proportion of reservoir-prone subtidal facies. Analysis of compensated-neutron log porosity indicates that subtidal-prone intervals may be broadly characterized as having less than 11% porosity, whereas intertidal-prone intervals generally exceed 11% porosity. From this observation, a simple computer algorithm allows facies interpretation within wells lacking core. Intertidal-subtidal facies ratio maps generated from this algorithm closely match the distribution of pre-Wolfcampian structural elements and present-day structure. Intertidal facies are more common across the crest of structural highs. Future drilling in the North Jenkins area should pursue structural-flank positions where subtidal facies are volumetrically more abundant. Care should be taken, however, to avoid completions below the composite oil-water contact. Subtidal facies are compartmentalized within individual sequences of a progradational sequence set. Progradational stacking and compactional drape over deeper structures produces discrete flow units with potentially independent oil-water contacts.

Depositional patterns following the 1870s avulsion of the Saskatchewan River (Cumberland Marshes, Saskatchewan, Canada)

ABSTRACT The avulsion of the Saskatchewan River (Cumberland Marshes, east-central Saskatchewan, Canada) has resulted in new floodplain deposition over an area of more than 500 km2 since beginning in the 1870s. The avulsion deposits are predominantly fine grained and could be otherwise classifiable as sediments in alluvial floodplains presumed to be supplied by intermittent flooding of dominant master channels. The North Angling and Cadotte channels are older pre-existing channels that were intercepted and appropriated by the expanding avulsion. Following annexation of the North Angling Channel, stable cohesive banks inhibited channel enlargement and crevassing, forcing the increasing discharges to be accommodated by overbank flow and in the process, enhancing new levee aggradation. These levee deposits coarsen upward and thin downstream as a result of greater competence and higher proximal deposition rates brought on by the larger flows. In contrast, the Cadotte Channel responded to increasing discharges by levee breaching, giving rise to splay progradation and floodbasin filling. The Cadotte splay complex, formed from the avulsively annexed Cadotte Channel, represents a small but typical area of the avulsion belt. Facies characteristics and interrelationships examined from auger borings and vibracores are presented to illustrate patterns of deposition and floodplain development. Avulsive flooding initially raised local water-surface elevations, deepening small existing lakes and creating new shallow (1-2 m deep) lakes on the floodplain surface. Deposition began when levees of the Cadotte Channel were breached, allowing water and coarser sediment to spread as sheet-like flows while finer sediment was carried and deposited farther into the basin. Later, as the proximal splay sheets coalesced and grew basinward, flows became channelized, and progradation of mouth bars into marshes and shallow lakes, together with deposition of interchannel fines, became dominant, forming coarsening-upward sequences. Further floodplain aggradation developed by overbank flooding when splay channels became stabilized and bordered by levees. Gradual abandonment was accompanied by deposition of organic-rich fine-grained sediment on top of the progradational sequences. Throughout the avulsion, the North Angling Channel has stood as an older alluvial ridge separating two broad areas of avulsive deposition. Its recent levee deposits now grade basinward into these new floodplain sediments composed of thicker heterogeneous, predominantly upward-coarsening, and rapidly deposited facies of the avulsion.

Mississippian carbonate ramp-to-basin transitions in south-central New Mexico; sequence stratigraphic response to progressively steepening outer-ramp profiles

ABSTRACT The sequence stratigraphic architecture of Mississippian strata from the Sacramento and San Andres Mountains, south-central New Mexico, illustrates the combined effects of relative sea-level change and progressively increasing depositional gradients on sedimentary processes and resultant stratal geometries along the outer parts of a carbonate ramp-to-basin transition. Mississippian strata can be subdivided into three progradational sequences (Sequences 1-3) and a fourth, basin-restricted, onlapping sequence (Sequence 4). Sequences 1-3 contain wackestone- to mudstone-dominated transgressive systems tracts that are overlain by packstone- to grainstone-dominated progradational systems tracts. Progradational systems tracts created distinct ramp-to-basin transitions (outer ramp margin) that are characterized by basinward stratal thinning and a concomitant change to muddy lithofacies. Sequence 4 consists of onlapping, fining-upward stratal packages of peloidal-skeletal grainstone that upward to laminated, spiculitic lime mudstone. The outer-ramp-margin facies of Sequences 1 through 3 gradually steepened the outer ramp profile until the cumulative effects of the steepening caused the depositional gradient to surpass an grade (i.e., the gradient at which deposition and erosion/bypass are about equal). Once the depositional gradient exceeded the equilibrium for the caliber of sediment supplied to the outer ramp margin, widespread submarine erosion and sediment bypass on the outer ramp was initiated, resulting in deposition of basin-restricted strata of Sequence 4. Similar relationships and inferred processes have been recognized in progradational siliciclastic shelf deposits and are collectively described by previous workers as slope readjustment. The Mississippian ramp-to-basin transition in south-central New Mexico illustrates how sediment deposition, dispersal, and erosion responded to evolving depositional gradients and in turn controlled resultant, large-scale stratal geometries.

Lithological control on the occurrence of chlorite in the diagenetic Wealden complex of the Bilbao anticlinorium (Basco-Cantabrian Basin, Northern Spain)

AbstractSandstones and shales from the southern flank of the Bilbao anticline in the Gordexola and Orozko valleys, northwestern Spain, have been subjected to a detailed sedimentological, mineralogical and geochemical study. They are composed of proximal frontdeltaic sandbars interbedded with silty-clayey sediments and correspond to a prograding sequence of a deltaic system. The clay mineral assemblages of doublets of sandstones and shales are composed mainly of illite, chlorite and illite-smectite mixed-layers R3 (ISII). The clay diagenesis consists of the transformation of smectite and I-S mixed-layers to illite, the precipitation of Fe-rich chlorite in the pore spaces, and the alteration of micas to chlorite. According to petrographic, mineralogical and geochemical analyses, chlorite is more abundant in sandstones than in shales in both the <2 µm and coarser fractions. The relative abundance of chlorite increases in the sandstones located at the top of coarsening upward sandbars. Furthermore, chlorite formation occurs preferentially in the coarser grained sansdstones previously cemented by ankerite. The geochemical and petrological investigations suggest that chlorite formed during burial diagenesis in a relatively closed system.

Palynostratigraphy and depositional history of the Palaeocene Ormesby/Thanet depositional sequence set in southeastern England and its correlation with continental West Europe and the Lista Formation, North Sea

Analysis of palynofloras from eleven sections of the Late Palaeocene Thanet Sand and Ormesby Clay formations of southeastern England has enabled construction of a stratigraphy for a period of approximately 2 Myr, based upon eighteen palynofloral association sequences. This high-resolution stratigraphy has allowed the accurate recognition of the extent of depositional sequences in the region. Deposition of claystones was initiated in Norfolk after 58.5 Ma as a prograding series of five parasequences, with the onset of sedimentation in the majority of the southeastern England area resulting from a subsequent rise in relative sea level at around 58 Ma. Relative sea levels reached a maximum at 57.91 Ma, after which a period of condensed sedimentation ensued. This first maximum flooding surface initiated deposition in the far south of the area (Kent), and was succeeded by a distinct fall in relative sea level and subsequent retrogradation of sedimentary units. A major change in lithology and palynofloras is marked by the initiation of a further sequence at approximately 57.5 Ma, with a second maximum flooding surface occurring shortly afterwards, followed by prograding sedimentary units. The uppermost part of the Thanet Sand and Ormesby Clay formations is composed of two separate maximum flooding events, the uppermost of which is poorly preserved. The lack of any other depositional units between these events suggests the presence of a significant disconformity at this point. Comparison of the palynofloral association sequences and flooding events of southeastern England, with sections in Belgium, Denmark and with the Lista Formation in the North Sea, demonstrates a close affinity between these areas. The transgressive surface identified at the base of the Thanet Sand Formation in southeastern England is preserved as the shales immediately overlying the Balmoral tuffite of the Lista Formation, and as the base of the Orp Sands in Belgium and Holmehus Formation in Denmark. The subsequent record of relative sea level change in the south is mirrored in the Lista Formation, with the maximum flooding events being represented by shales and the intervening prograding sequences by siltstones and thick sands. This comparability of the record implies a similar regime of relative sea level change in the North Sea and southeastern England area throughout the deposition of the Lista/Ormesby/Thanet sediments, prior to the contrasting environments of Sele Formation-Lambeth Group sedimentation.

Abstract: Trace Fossil Shallowing-Upwards Stacking Pattern of High-Order Progradational Sequences: An Example from the Cretaceous-Tertiary of Western Patagonia, Argentina.

Abstract: Trace Fossil Shallowing-Upwards Stacking Pattern of High-Order Progradational Sequences: An Example from the Cretaceous-Tertiary of Western Patagonia, Argentina.

Lithological Control on the Occurrence of Chlorite in the Diagenetic Wealden Complex of the Bilbao Anticlinorium (Basco-Cantabrian Basin, Northern Spain)

AB ST R ACT : Sandstones and shales from the southern flank of the Bilbao anticline in the Gordexola and Orozko valleys, northwestern Spain, have been subjected to a detailed sedimentological, mineralogical and geochemical study. They are composed of proximal frontdeltaic sandbars interbedded with silty-clayey sediments and correspond to a prograding sequence of a deltaic system. The clay mineral assemblages of doublets of sandstones and shales are composed mainly of illite, chlorite and illite-smectite mixed-layers R3 (ISII). The clay diagenesis consists of the transformation of smectite and I-S mixed-layers to illite, the precipitation of Fe-rich chlorite in the pore spaces, and the alteration of micas to chlorite. According to petrographic, mineralogical and geochemical analyses, chlorite is more abundant in sandstones than in shales in both the <2 mm and coarser fractions. The relative abundance of chlorite increases in the sandstones located at the top of coarsening upward sandbars. Furthermore, chlorite formation occurs preferentially in the coarser grained sansdstones previously cemented by ankerite. The geochemical and petrological investigations suggest that chlorite formed during burial diagenesis in a relatively closed system.

Phase relations among smectite, R1 illite-smectite, and illite

A variety of smectitic and illitic clays were studied by TEM and AEM, following ex- pansion by L.R. White resin, to define phase relations for clay minerals undergoing di- genesis and low-grade metamorphism. Samples included a prograde shale sequence from the Gulf Coast, a hydrothermal bentonite from Zempleni, Hungary, and shales from the Nankai Trough, Japan, Michigan, and the Welsh sedimentary basin. All samples were dominated by various proportions of only three kinds of clay minerals: smectite having no 10 Alayers, R1 illite-smectite (I/S) (50% illite), and illite with only small proportions of smectite-like interlayers; mixed-layer phases with intermediate ratios of I/S were observed only as minor components of other layer sequences. Lattice fringe images show that the common layer spacing of R1 I/S is 21 Aand that it has 0.7-0.8 K pfu; its properties are not an average of those of smectite and illite, which is consistent with the uniqueness of the R1 I/S structure. A prograde sequence of clay mineral transitions in the studied samples can be charac- terized by five stages with different combinations of the three major phases (i.e., smectite, R1 I/S, and illite) corresponding to different grades. The sequence from low to high grade is (1) pure smectite, (2) smectite with small proportions of discrete R1 I/S and illite, (3) R1 I/S with small proportions of smectite and illite, (4) illite with some R1 I/S and smectite, and (5) illite. Common exceptions to this scheme are inferred to be caused by the inherent metastability of all phases, in occurrences that are rate or path dependent.