ABSTRACT
Very fine sandstone and mudstone facies of the Upper Triassic Dolores Formation of southwestern Colorado provide evidence for shallow lacustrine deposition coeval with clastic lacustrine systems of the Dockum Group in Texas. Dolores and Dockum lakes had frequent water-level fluctuations; however, Dolores lakes were filled primarily by shoreline sequences, in contrast to the delta-filled Dockum lakes.
Typical Dolores shoreline sequences fine upward, are laterally continuous over 3 km, are 5 to 15 m thick, have sharp nonerosional planar bases, and grade upward from very fine sandstones into mudstones. The very fine sandstones contain wavy, 1-cm thick bedding; low-angle trough and planar tabular cross-bedding; and isolated symmetrical channels. The overlying silty mudstones are commonly intensely bioturbated. These fine-grained shoreline deposits suggest that weak longshore currents distributed sand away from distributary mouths, and/or that sediment was transported by flow across the low-gradient, lake-margin plain.
Frequent subaerial exposure of these shoreline sequences is documented by abundant desiccation-cracked and rain-textured mudstone drapes, and by well-developed caliche profiles. During low stages of Dolores lakes, distributary channels locally prograded across and sometimes incised into the shoreline sequences. These symmetrical channels, 8 to over 50 m wide, commonly contain basal mudstone-clast/caliche-pebble conglomerates and were abandoned episodically as indicated by alternating beds of very fine sandstone and mudstone. Lake-edge distributary channels contain fillings of wave-reworked, wavy-bedded to rippled very fine sandstone.
