Likely Counter Point Bars in Coarse-­‐grained Meandering Fluvial Deposits in the Brushy Basin Member of the Jurassic Morrison Formation


Fluvial deposits in the Tithonian Brushy Basin Member of the Morrison Formation were studied in order to create a facies architectural diagram and reconstruction of a fluvial system exposed in outcrop. This study provides an opportunity to study a coarse-grained meandering fluvial system, and provides one of the first descriptions of an ancient counter point bar in outcrop. Eleven measured sections across a 300m long outcrop were correlated on a photomosaic, creating bedding diagrams showing facies architecture and grain-size variations. The main channel facies shows a fining up succession from pebble to medium-grained sandstone, overlain by the fine sandstone and siltstone of the upper point bar. Four main facies were described, including a silty, mud-rich floodplain facies, a sandy thin laterally continuous facies showing convoluted bedding interpreted as overbank splays, a sandy, trough-cross-stratified channel fill facies and a silty, mud-rich counter point bar/ flood drape/ abandoned channel fill facies. Channel properties were assessed using the cross-sectional area of the channel and bedforms to discharge correlation. The channel was assessed to be 25.2m wide with an average channel depth of 5.6m. A flow discharge averaging 55 m3/s was calculated by using the law of the wall to find the average flow velocity of water through multiple rectangles covering the channel cross-section. Initial results show the main outcrop to represent a single meander loop initially expanding as the fluvial channel flowed north, then changing into eastward translation followed by further expansion. The deposits from the eastward translation of the fluvial channel show an eastward paleocurrent, concave bar deposits in the distal end of the meanderscrolls and are rich in silt- and mudstone, forming non-tidal, inclined heterolithic stratification, indicating a likely counter point bar deposit. Counter point bars (CPBs) originate as a meander scroll migrates laterally and encounters an obstacle in the form of an erosion-resistant substrate, forcing it to begin longitudinal translation. Thus the CPB is always located on the distal end of the point bar, before the cutbank.



Morrison Formation, Brushy Basin Member, Coarse Grained Meandering Fluvial System, Counter Point Bar, Utah