Symmetry, Stratigraphy, and Petrography of Cyclic Cretaceous Deposits in San Juan Basin

ABSTRACT Late Cretaceous strata of the San Juan basin consist of cyclically interstratified non-marine, nearshore marine, and offshore marine clastic sediments which were deposited both during marine transgressions and regressions. Thickness of the transgressive and regressive parts of these cyclic sequences varies, permitting subdivision into two types of cycles: symmetrical and asymmetrical. In symmetrical cycles the thickness of transgressive and regressive parts are nearly equal; in asymmetrical cycles the transgressive sandstone is absent. The Hosta-Point Lookout wedge is an example of a symmetrical cycle. At its base the transgressive marine Hosta Sandstone overlies continental strata of the Crevasse Canyon Formation. The Hosta Sandstone grades upward into the offshore marine Satan Shale. The Satan Shale marks the midpoint of the cycle and the maximum marine inundation; it grades upward into the regressive marine Point Lookout Sandstone. The Point Lookout is overlain by the continental Menefee Formation. Southwestward, toward the former shoreline, the Satan Shale pinches out and the transgressive and regressive sandstones merge into a single massive sandstone, which still farther southwestward grades into continental strata. The Mulatto-Dalton cycle is asymmetrical for it lacks a basal transgressive sandstone. Instead, the offshore Mulatto Shale directly overlies the continental Dilco Coal with only scattered marine sand lenses at the contact. The Mulatto Shale grades southwestward (toward former shoreline) and upward into the regressive marine Dalton Sandstone which in turn grades southwestward into, and is overlain by, continental deposits of the Crevasse Canyon Formation. Petrography is closely related to the sandstone depositional environments as follows. Regressive sandstone type.—Upward increase in maximum and median grain diameter; upward decrease in abundance of primary dolomite grains. These vertical changes reflect progressive shallowing of the basin during regression. Transgressive sandstone type.—Upward decrease in maximum and median grain diameter; upward increase in abundance of primary dolomite grains. These vertical changes reflect progressive deepening during transgression. Non-marine sandstone type.—Wide range of grain sizes; primary dolomite grains absent; abundant carbonaceous material; higher proportion of non-quartz detritus. Petrography and the cycle concept are useful for recognizing and solving correlation problems.