Estuarine Sandstone Filling Tidal Scours, Lower Cretaceous Fall River Formation, Wyoming

Abstract

ABSTRACT To obtain comparative data for identifying other tidal deposits, we studied the attributes of ancient estuarine sandstone bodies that filled tidal scours into the mouths of small streams. These estuarine sandstones in the Lower Cretaceous Fall River Formation, northeastern Black Hills, Wyoming, form seaward-imbricated complexes in which each younger body fills scours into older ones. Both the complexes and occasional isolated estuarine bodies are enclosed in tidal-flat deposits or their adjacent seaward equivalents. From the thicker landward ends of each scour filling, fluvial sandstone grades seaward into upper estuarine and then lower estuarine sandstone. Fluvial sandstone consists of the vertical sequence (1) basal cross-laminated trough beds, (2) middle parallel beds composed of either low-angle cross laminae or laminae parallel with bedding surfaces, and (3) upper wavy parallel beds composed of a variety of current-ripple types. Also, fluvial sandstone exhibits triple grain-size grading with an upward decrease in grain size within (1) each cross-laminated trough bed, (2) each vertical sequence, and (3) each succession of superposed partial vertical sequences that form the fluvial sandstone body. Upper estuarine sandstone is characterized by high-angle seaward-dipping cross laminae between arallel bedding surfaces. Lower estuarine sandstone contains both large- and small-scale truncated wave-ripple laminae between wavy parallel bedding surfaces. Although no exact modern counterparts of these estuarine sandstones are known to the writers, such sandstone bodies could form by filling tidal scours like those present on the German coast north of the Elbe River.