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.

Abstract The purpose of this field trip is to provide an overview of Miocene basin development in the Lake Mead region, demonstrate how basin-fill deposits reflect tectonic activity on a variety of structures, and highlight the work of Ernie Anderson in this region. The Basin and Range province is superb for the study of major normal and strike-slip fault systems that accommodate large-magnitude extension. Within this province, the Lake Mead region provides exceptional exposures of synextensional Miocene basins and faults and is a transition zone between predominantly half-graben–style basins and ranges to the north and the highly extended Colorado River Extensional Corridor to the south. The region also embraces a change from thick Phanerozoic sedimentary rocks in the north to Precambrian crystalline basement rocks overlain by late Tertiary volcanic rocks in the south. The early Paleozoic “Cordilleran hingeline” and the southeast margin of east-directed Mesozoic thrusting are also within this transition zone, but the area contains a strong overprint of late Tertiary tectonism. This overprint is strongest near the intersection between two major strike-slip fault systems: the right-lateral Las Vegas Valley shear zone and the left-lateral Lake Mead fault system. Miocene sedimentary rocks record the onset of major extension and the development of numerous, complex structures. Details of the extension and the resulting complexity are not fully understood and many issues remain unresolved. The relative importance of normal versus strike-slip faults is debated as are the details of how and why faults develop through time.