ABSTRACT Different depositional environments (fluvial, deltaic, lacustrine, tidal-marine) have been proposed for the Upper Miocene to Pliocene Madre de Dios Formation exposed in the upper reaches of the Amazon River catchment in the Andean retroforeland region. This study constrains the stratigraphy, depositional environment, and drainage evolution in southwestern Amazonia through petrographic and provenance analysis of the sand and mud fractions of the three recognized members (A–C) of the Madre de Dios Formation at three stratigraphic sections measured previously along riverbank outcrops: Cerro Colorado, Piedras River, and Candelaria. Petrographic analyses of thin sections of sand separates from 32 sandy samples showed them to be litho-quartzose to quartzo-lithic in composition, with variable feldspar content and a recycled-orogen provenance. Sand components were predominantly monomineralic to polymineralic quartz, and sedimentary and metamorphic lithic fragments. Muscovite, potassium feldspar, plagioclase, and volcanic lithics were less abundant. These sand components are consistent with derivation from the Andean range to the west. Quartz and feldspar content generally increased up section from member A to member B. Sand composition in member C is similar to the modern river sand composition, consistent with recycling of Madre de Dios Formation sand into the modern river system. Petrographic analyses of 144 smear slides of the mud fractions showed no significant changes in silt composition, i.e., mainly quartz, feldspar, and mica, among the three members. X-ray diffraction of eight mud samples showed their clay mineralogy to be dominated by kaolinite and illite, with some smectite and chlorite in member B. None of the 144 smear slides or the 32 thin sections contained marine or marginal-marine biogenic debris. Mud-rich samples from the Madre de Dios Formation exhibit six main colors that characterize distinct intervals (designated I to VI) that occur in the same stratigraphic order in each measured section, from I at the base to VI at the top. The boundaries of these intervals do not correspond directly to member (A, B, C) boundaries; therefore, the colors are at least partly secondary. If primary, the red to orange to brown mudstone (which is dominant in Madre de Dios Formation members A and C) would suggest development in oxidizing environments, consistent with fluvial systems. Based on its light olive-gray color and smectite content, interval IV in member B may have been deposited in, or subjected to, a more reducing environment, such as a lacustrine-deltaic setting, with low-lying topography and poor drainage. In sum, the Madre de Dios Formation exhibits up-section compositional and thickness trends that are consistent with changes in depositional environment from fluvial (member A) to lacustrine/deltaic (member B) to fluvial (member C), as proposed by previous workers.

Abstract The Middle Jurassic–Lower Cretaceous in the eastern Dutch offshore provides excellent examples of sand-rich sediments that locally accumulated in the vicinity of rift basin margins affected by salt tectonics. These types of deposits are often geographically restricted and difficult to identify, but can be valuable targets for hydrocarbon exploration. The distribution, thickness and preservation potential of fluvio-lacustrine, shallow- and deep-marine sediments is discussed to provide new insights into the regional and local tectonostratigraphy of the Dutch Central Graben, the Terschelling Basin and their neighbouring platforms. New sedimentological, geochemical, biostratigraphic, stratigraphic and structural information have been analysed and integrated into a new tectonostratigraphic model for the Callovian Lower Graben Formation, Oxfordian Middle and Upper Graben formations, Early–Middle Volgian Terschelling Sandstone and Noordvaarder members, and the Late Volgian–Early Ryazanian Scruff Greensand Formation. It is demonstrated that salt withdrawal at the basin axis was the primary control on the generation of high accommodation during the Callovian–Early Kimmeridgian. Incised valleys developed on the platforms providing lateral sediment input. During the Late Kimmeridgian–Ryazanian salt migration shifted laterally towards the basin margins, providing accommodation adjacent to active salt bodies and deposition of overthickened sandy strata.

Abstract The Kayenta Formation, Warner Valley, Utah, shows lateral and vertical clustering of mud-delta propagating-channel sand bodies within a matrix of fine-grained open-lake deposits and further provides opportunity to develop a fluvio-deltaic depositional-process model. Clustering due to nonrandom stream avulsion is well documented for high-accommodation fluvial systems operating in alluvial plains but not well established for lacustrine systems with abundant fluvio-deltaic lobes. Kayenta Formation delta lobes have similar spatial clustering to those observed in fluvial channel belts and possibly extend this clustering concept to fluvio-lacustrine systems. Lithofacies were mapped on three large photo panoramas, and architectural-element analysis was used to identify bounding surfaces of fluvial channel-deltaic lobes. Fluvio-lacustrine delta lobes reflect linear channels that propagate across mud deltas with negligible bifurcation and generate fluvial channel belts incised into lake mudstone. Channels are associated with thin sand sheets or “blow-out wings” that extend multiple channel widths from the channel and cover levee and mud-delta deposits, but delta-front sand beds are absent. The stages of evolution for these propagating channels is preserved in the variation of channel-lobe architecture and reflects mud-delta propagation at the mouth of each channel in the absence of delta-front sand. Sand is outpaced by mud in the ever-lengthening channel, which reduces sand at the channel mouth and diminishes necessity for channel bifurcation. The resulting deposit is thus a frontal mud-delta lobe bisected by a later single sandy channel belt with lateral sand wings. Statistical analysis of these channel belts shows clustering. Clustering of fluvial bodies within shallow lakes is significant in predictive reservoir models because it improves connectivity and localization of delta-lobe reservoirs. The clustering of delta lobes in fluvio-lacustrine systems is theorized to reflect the preferential channel avulsion centered on the axis of the primary channel feeding into the lake and preferential avulsion fairways of feeder channels. Both the segregation of sand and mud though channel lengthening and the clustering are explainable in fully autocyclic terms. The needed allocyclic driver to trigger these fluvio-lacustrine processesis an accommodation rate sufficiently low compared with lake filling rate as to maintain shallow-water conditions across the lake system through multiple generations of channel propagation.