Red Bed Members of the Lower Triassic Moenkopi Formation, Southern Nevada: Sedimentology and Paleogeography of a Muddy Tidal Flat Deposit

Abstract

ABSTRACT The Lower Triassic Moenkopi Formation in southern Nevada and on the adjacent Colorado Plateau represents a gradual transition from continental fluvial sedimentation in Arizona through intertidal and shallow marine deposition in western Nevada and Utah. The red bed members in Clark County, southern Nevada, are muddy tidal flat deposits. Modern analogies are the upper surface of the Colorado River Delta and the Hwang Ho Delta. Analysis of textures and small-scale sedimentary structures in the red beds has led to the recognition of seven distinctive lithofacies: (a) laminated mudstone, (b) medium- to very thick-bedded sandstone and siltstone, (c) thin- to medium-bedded siltstone and sandstone, (d) turbated sandstone to mudstone, (e) gypsum, (f) limestone, and (g) dolomite. Lithofacies a as deposited on seaward-sloping, high intertidal to subtidal mud flats. Lithofacies b and c represent relatively high energy beach, intertidal channel, and subtidal bar deposits. Lithofacies d was deposited in subtidal to supratidal environments, depending upon the cause of turbation. Lithofacies e and g were deposited in supratidal or sabkha environments. Lithofacies f was deposited in tidal channels and on tidal flats as bars, shoals, and washover fans. Two sedimentation sequences are evident in the red bed members. A 15 m thick tidal channel--mudflat--tidal channel sequence, consisting of thin-bedded sandstone overlain by laminated mudstone then thin-bedded sandstone. formed by lateral migration of tidal channels. A 1 m thick supratidal--high intertidal flat sequence, consisting of turbated gypsiferous mudstone overlain by laminated mudstone then thin-bedded siltstone, formed by lateral shifts of depositional environments in response to minor shoreline fluctuations. The depositional basin was in a tectonically stable area of very low relief and arid climate. Depositional environments across the broad intertidal mud flats shifted in response to small changes in eustatic sea level and rates of subsidence and sedimentation. East of the mud flats, extensive sabkhas graded into continental fluvial environments, while more open shallow marine environments occurred to the west. The lithofacies and sequences observed in the red bed members provide a framework for a facies model of a muddy tidal flat complex.