Anatomy of a transgressive lag: Panther Tongue Sandstone, Star Point Formation, central Utah

The Panther Tongue of the Star Point Formation in central Utah contains a variety of transgressive lag deposits that, when mapped regionally, show a sensitive dependence upon pre‐existing topography of the palaeoshoreline. The Panther Tongue consists of a coarsening‐upward sandstone wedge that prograded into the Western Interior Seaway during Late Cretaceous (Santonian) time. High‐resolution sequence stratigraphic analysis revealed that this member was deposited during the long‐distance (>50 km) regression and transgression of a delta into shallow‐marine environments, containing basal highstand, forced regression, lowstand and transgressive systems tracts. Based on grain size, clast composition, lateral extent and stratigraphic position, the coarse sandstones on top of the Panther Tongue were classified into four types: (1) simple; (2) dispersed; (3) oxidized; and (4) local lags. The simple lag is composed of dark grey coarse sandstone with oyster fragments and shark teeth. This lag is typically extensively bioturbated and massive. Laminated and cross‐bedded units are also common. This type of coarse sandstone is interpreted as a typical transgressive lag. The dispersed lag differs in that it contains abundant mud and commonly occurs as multiple beds in thick intervals of muddy sandstone. Mixing of bay/estuarine and shallow ‐ marine mud with simple lag sand may be responsible for deposition of this type of coarse sandstone. The oxidized lag is distinctive in its reddish colour with extensive bioturbation and is commonly overlain by a simple lag. The local lag is composed of thin‐bedded, dark grey, coarse sandstone, occurring locally between the mouth bar and distributary channel. The variation in types, grain size and bed thickness of the coarse‐grained lags was mainly controlled by antecedent topography as suggested by immediately underlying lithofacies. Relatively thick (≈30 cm) simple lags are present on top of mouth‐bar sandstones, whereas dispersed lags are common on top of the distributary channel sandstone and in bay/estuarine and shallow‐marine mudstones. Erosion of topographic highs (mouth bar) resulted in relatively thick accumulation of simple lags. In topographic low areas such as distributary channel, estuary, bay and shallow‐marine environments, fine‐grained muddy sands that were eroded from the nearby topographic highs were redeposited. Intermittent storm waves transported coarse sands both landward and seaward, forming a dispersed lag. The net effect was reworking of local topographic relief during overall transgression, forming an apparently planar transgressive surface of erosion.