Laserfiche WebLink
sinoides are th.e most common, both indicating littoral to <br /> • <br /> shallow neritic. environments , and O.phiomorpha , a littoral <br /> indicator . The marine unit ranges from 90'-200 ' in thickness . <br /> The Middle Sandstone Submember , which overlies <br /> the Lower Bowie Shale Submember , ranges in thickness from <br /> 30' -130' , and is marine in origin, fine-to-medium grained , <br /> quartzitic to feldspathic , poorly to moderately sorted , with <br /> a clay matrix at the top , and buff to white in color . Sub- <br /> angular grains of black chert occur . <br /> The Middle Sandstone is similar to the Rollins , <br /> but is generally more massive, with cross-bedding uncommon. <br /> Where bedding does occur , subparallel laminations predominate. <br /> Where cross-bedding is present , a transport direction to the <br /> • south is indicated . <br /> The only observed fossils were several inter- <br /> vals of well-developed Ophiomorpha . The upper surface of <br /> the sandstone is usually rough and irregular , and commonly <br /> displays a thin limmonitic coating . In a few cases continuous <br /> deposition from marine sandstone to the overlying nonmarine <br /> sequence indicated by transition into the overlieing member , <br /> but such occurrences are not common. Some probable root <br /> structures are present in the upper foot of the sandstone , <br /> along with. considerable coalified plant debris . <br /> The variable thickness of the unit is probably <br /> due to differing rates of subsidence and sediment influx , <br /> and to differing transport abilities of the currents around <br /> • <br /> 23 <br />