Cyclic Sedimentation Within Point Lookout Formation (Upper Cretaceous)--A Model for Strandline Progradation and Sandstone Distribution: ABSTRACT

Abstract

The regressive marine Point Lookout Sandstone in the southeastern San Juan basin, New Mexico, was deposited under conditions of relatively low wave energy, moderate tidal strength, and locally moderate fluvial input. Two idealized coarsening-upward vertical profiles characterize the preserved lithologic record. The first profile, documenting strike-aligned sandstones dominated by wave processes, is composed of the upward progression: offshore mudstone--offshore transition mudstone to sandstone--lower shoreface sandstone--upper shoreface sandstone--estuarine sandstone. The second vertical profile, reflecting local fluvial input and dip-aligned sandstone bodies, is typified by the upward progression: offshore mudstone--offshore transition (prodelta) mudstone to sandstone--d lta-front sandstone--estuarine and tidal distributary sandstone. In both profiles, subaerial beachface and shallowest marine lithofacies are rarely preserved due to erosion and replacement by estuarine sandstones during seaward strandline migration. Basinwide regression of the Point Lookout formation consists of numerous small-scale asymmetric transgressive-regressive cycles. Mappable erosive and/or non-depositional surfaces characterize transgressive events which separate genetically equivalent progradational sandstones. Duration of these small-order cycles is broadly estimated in the range of 104 years, probably at less than 50,000 years each. Cycle geometry is dependent upon the interplay of sedimentation rate and submergence, such that sedimentation rate controls the down-dip progradational sandstone width, and submergence influences the total cycle thickness. Because differences in thickness within the overall Point Lookout formation result from variability in cycle overlap (stacking), an understanding of cycle ge metry allows prediction of local stratigraphic pinchouts which may serve as stratigraphic traps for hydrocarbons. End_of_Article - Last_Page 541------------