Basal Portion of the Travis Peak Formation at the Base of the

Cretaceous in Southwestern Lampasas County, Texas

At the beginning of the Cretaceous Period, the sea advanced from the south and east and eventually covered the entire Central Texas area.  This major transgression and several minor regression along with an oscillating shoreline influenced the deposition of the sequence of sediments known as the Travis Peak Formation.  The Travis peak has been divided into seven members which do not have regional lateral extent. The basal Travis Peak unit discussed herein has been termed the Sycamore Member.

The basal portion of the Cretaceous age Travis Peak Formation, Trinity Group, in southwest Lampasas County, Texas does not exceed 50 feet.  About 20 to 25 feet of the basal portion  of the Travis Peak includes a conglomerate. A packsand with associated material, overlies the conglomerate and makes up the remaining portion of the section to 50 feet.  This upper half contains the more calcareous sandstone, limestone and marly material.  The Travis Peak onlap the eroded Paleozoic surface and dip at a few feet per mile as opposed to the Paleozoic rocks dip up to 100 feet per mile. The basal conglomerate forms this angular unconformably with the underlying Paleozoic rocks of the Lower Pennsylvanian Marble Falls and Smithwick formations. 

The Travis Peak conglomerate is composed  mainly of  gravel size Pennsylvanian limestone, quartz, flint (probable Marble Formation) and other Paleozoic material, cemented in a matrix of sandy to clayey limestone (micrite).  Small fragments of sand and gravel are present in the matrix, which in some cases is iron stained and very hard.  The whole mass sometimes has a mottled appearance from the variously colored materials composing it.  The colors are pink, red, yellow, purple, brown and several shades of blue.  The materials of the conglomerate vary in size from fine gravel to boulders size. Their form indicates subjection to different degrees of abrasion.  Most are subangular, while others approach a well rounded texture.  

The packsand varies in texture from a compact, nonindurated, yellowish mass of coarse sand, quartz, and chert pebbles to a close-grained, stratified, calcareous sandstone, brown to whitish in color.  Strata of calcareous sand occur in thin variable beds.

There is a general stratification in these basal beds and thin limestone beds may be present in the basal section.  The materials composing the conglomerate and packsand are not well sorted as to size and generally have a subangular texture.  This coarse material gradually becomes less conglomeratic higher in the section, giving way to a mass of loosely bedded very fine gravel to coarse sand (packsand), lime marl and calcareous sandstone. Horizontally, the lithology is fairly consistent until interrupted by adjacent highs or lows in the underlying eroded surface.

 

The following photos show some of the outcrops of the basal Travis Peak and rock samples of the conglomerate in southwestern  Lampasas County Texas.

 

Basal Travis Peak Formation 2 miles west of Lampasas, Texas on CR1255.

Exposure of fine limestone and chert gravel conglomerate in micrite matrix.

 

Basal Travis Peak Formation 2 miles west of Lampasas, Texas on CR 1255.

Exposure of fine to coarse limestone and chert gravel conglomerate in micrite matrix.

 

Basal Travis Peak Formation 2 miles west of Lampasas, Texas on CR 1255

Exposure of fine to coarse limestone and chert gravel conglomerate in micrite matrix.

 

Basal Travis Peak Formation 2 miles west of Lampasas, Texas on CR 1255 - Fine, subangular, poorly 

sorted limestone with some chert gravel Conglomerate with  coarse gravel in a sandy micrite matrix.

 

Basal Travis Peak Formation 2 miles west of Lampasas, Texas on CR 1255 – Fine, subangular, poorly

 sorted limestone and chert gravel Conglomerate in a  micrite matrix

 

Basal Travis Peak Formation 3 miles northeast of Bend, Texas on CR 581– Coarse to fine, subangular,

 poorly sorted limestone  and chert gravel Conglomerate in a  micrite matrix