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Middle to Upper Cambrian Rocks of the Llano Region, Texas

Dr. Emilio Mutis-Duplat, Professor of Geology and Chair Department of Science and Mathematics, College of Arts and Sciences, The University of Texas of the Permian Basin, Odessa, Texas

 

The Precambrian metamorphic and igneous rocks of the Llano region, Texas are unconformably overlain by the Cambrian sedimentary rocks of the Riley Formation, which is subdivided from bottom to top into three members:  Hickory Sandstone Member, Cap Mountain Limestone Member, and Lion Mountain Sandstone Member.  The Riley Formation is disconformably overlain by the Cambrian sedimentary rocks of the Wilberns Formation, which from bottom to top is subdivided into four members:  Welge Sandstone Member, Morgan Creek Limestone Member, Point Peak Member, and San Saba Member.

    

The Hickory Sandstone Member of the Riley Formation has been subdivided in the field into three units:  lower, middle, and upper.  The lower unit rests unconformably on metamorphic or igneous Precambrian rocks.  This lower unit is generally poorly exposed, tan to light brown, coarse grained, and slightly conglomeratic with subrounded pebbles.  The middle unit is the best exposed because of its resistance to erosion.  The sandstone is mostly various shades of yellowish to dark brown, thinly bedded, argillaceous, silty, and locally micaceous.  At the top the siliceous cement is gradually replaced by iron-oxide cement.  The upper unit is poorly exposed but easy to recognize in the field because of the dusky-red color of both the sandstone itself and the soil developed over it.  It is massive to thinly bedded, medium to coarse grained, iron cemented, and friable.  The quartz grains are exceptionally well rounded.  The Cap Mountain Limestone is composed of an alternation of silty and sandy limestone, light tan, dark gray, reddish brown, or dark brown.  It is generally fine grained and locally glauconitic pellets are abundant.   Limonite staining and replacement of glauconite by limonite are the common products of weathering.  The Lion Mountain Sandstone Member is composed of fine- to coarse-grained, dark-green, glauconitic sandstone, purplish sandy limestone, and calcareous fossiliferous coquina lenses.  The rock is moderately sorted.  Abundant nodules composed of hematite, believed to be the weathering product of glauconite, are found scattered over the surface of the sandstone.  The coquina lenses are composed of trilobite fragments and brachiopod shells.

    

The base of the Wilberns Formation is a disconformity that can be observed at many places, characterized by a thin limonitic and earthly zone of Lion Mountain Sandstone.   “The Welge Sandstone Member is a coarse- to medium-grained, mostly pale to dark yellowish-brown, typically nonglauconitic, sparsely fossiliferous, well-sorted, quartz, marine sandstone.  The grains are characteristically reconstituted and glitter in the sunlight” (Barnes and Bell, 1977, p. 25).  The base of the Morgan Creek Limestone Member is a sandy, light-gray to purplish, fine- to coarse-grained, fossiliferous, and glauconitic limestone.  Bedding is in general massive, and locally some beds are fossiliferous.  Close to the top  blue-green algae bioherms are commonly found.  The base of the Point Peak Member is located immediately below the first flat-pebble conglomerate found in the section.  The main components of the Point Peak Member are siltstone, conglomerates, shales, and blue-green algae bioherms.  Some siltstones are calcareous and dolomitic.  The base of the San Saba Member is located at the top of the last flat-pebble conglomerate found in the section.  The San Saba Member is composed of grayish-yellow to yellowish-brown, fine- to medium-grained and somewhat dolomitic limestone.  A few thin beds of glauconitic limestone are found.  Blue-green algae bioherms are very common.  Also present are silty limestone, limey siltstone, detrital limestone, algal limestone beds, and dolomitic limestone with glauconite at the top.  The San Saba Member is conformably overlain by the Ordovician Ellenburger Group.

 

Barnes, V.E., and Bell, W.C., 1977, The Moore Hollow Group of Central Texas:  The University of Texas at Austin, Bureau of Economic Geology Report of Investigations 88, 169 pages.