Sequence Stratigraphy of Dinosaur Lake: Small Scale Fluvio-Deltaic Stratal Relationships of a Dinosaur Accumulation at the Aaron Scott Quarry, Morrison Formation, San Rafael Swell, Utah

Swimming through the substrate: the neoichnology of Chalcides ocellatus and biogenic structures of sand-swimming vertebrates

“…Stratigraphic studies of the Aaron Scott Site (Figure 1) in central Utah indicate that this site was located on a delta along the margin of a large inland lake (Jeffery et al, 2011). Previous studies of the Morrison Formation indicate that groundwater fed marshlands occur and carbonate lacustrine environments have been described in lowlands, especially in Colorado (Jackson, 1979;Lockely et al, 1986;Dunagan, 1998;Dunagan and Turner, 2004).…”

“…The wellexposed strata of the Brushy Basin Member at the Aaron Scott Quarry shows patterns consistent with a prograding delta system in which sediments were deposited during the progressive fill of a large lake. Bone accumulations of this sort are previously unrecognized in the Morrison Formation and are rare for vertebrate assemblages in general, and it has been suggested that fluctuations in base level are the major factor responsible for the accumulation and preservation of bones at this site (Jeffery et al, 2011). The occurrence of vertebrate accumulation in relation to this lake basin fill provides a unique opportunity to characterize the taphonomic patterns of a lacustrine delta environment.…”

“…The eastern margin is steeply dipping and the western margin is more gently dipping (Stokes, 1986), and eroded into numerous cuestas. Beds dip approximately 5 degrees to the west, although it is difficult to determine how much of this is structural and how much is stratigraphic (Jeffery et al, 2011).…”

“…The Salt Wash Member is primarily braided stream and floodplain deposits sourced from surrounding mountain ranges. The Brushy Basin Member consists of a lower gray unit, representing lacustrine environments, that interfingers with the underlying Salt Wash Member and an upper red, gray and green variegated unit representing deposits on a broad, flat alluvial plain consisting of extensive floodplain deposits with ponds and swamps as well as alkaline lakes (Dodson et al, 1980;Demko et al, 2004;Jeffery et al, 2011). Calcareous nodular paleocalcretes are common within the floodplain deposits presenting further evidence of semi-arid conditions (Demko et al, 2004).…”

“…The quarry exposure is 15 m by 15 m and does not represent the entire length of the lake margin. The thickness of the deltaic units indicates 15 m of accommodation space in the lake (Jeffery et al, 2011), and lake levels fluctuated within this basin depending on climate variations. Tectonic uplift of the Colorado Plateau (Steiner, 1998) could have produced a basin in the area of the San Rafael Swell.…”