Wet/dry, terminal fan-dominated sequence architecture: A new, outcrop-based model for the Lower Green River Formation, Utah

Terminal fans developing on the alluvial plain along the edge of lacustrine basins can accumulate thick, sand-rich deposits that exhibit specific and unusual facies associations, channel-fill styles and stratigraphic architecture.; Terminal fans develop in relatively dry climates and are mostly active during violent, episodic storms. Facies and facies associations in the studied part of the lower Green River Formation (GRF) indicate high-energy, intermittent and rapidly-waning clastic deposition, as evidenced by abundant upper flow-regime, plane-parallel lamination, mud cracks developed along channel thalwegs and deep insect burrows within channel-fills. The high-energy and episodically-active character of the GRF terminal fans led to fan channels infilling with unusual architectural elements, such as large-scale, channel-wide concave-up sheets (CUS).; Within the study area, the GRF accumulated on the fringe of Eocene Lake Uinta, under relatively dry conditions. Dry climatic half-cycles led to the creation of a wide and dry lake-plain in the wake of the retreating lake. As the subsequent wet half-cycle started, drainage systems began to reestablish themselves on the lake plain. Because the dried-out, low water-table lake plain was wide, most drainages lost all their discharge to subsurface infiltration before reaching the lake, thus creating terminal fans on the lake-plain.; The lack of transition between lacustrine deposits and terminal fan deposits in the GRF indicates that the two systems were usually spatially segregated, being active in different zones of the basin during the same time interval. This segregation led to the accumulation of bipartite depositional sequences, composed of a lower, clastic part formed by prograding terminal fans during the early stages of the wet half-cycle, and an upper (mostly carbonate) lacustrine interval, accumulated during the later stages of the wet cycle and the early stages of the subsequent dry half-cycle. Early-stage terminal fan progradation is evidenced in most GRF sequences by the juxtaposition of distal, low-energy fan deposits directly onto the sequence boundary capping the desiccated lake plain formed during the previous cycle, and by the subsequent upward increase in channel size, which is the consequence of more proximal, higher-energy zones of the fan prograding over the early distal fan.