• BUREAU OF ECONOl\UC GEOLOGY THE UNIVERSITY OF TEXAS AT AUSTIN AUSTIN, TEXAS 78712 W. L. Fuaza, Director . fl) GEOLOGIC QUADRANGLE MAP NO. 51 • '1l U'.l >< • Q) t::J 8 Geology of the Hammetts Crossing Quadrangle, Blanco, Hays, and Travis Counties, Texas ~ ~ BJ <'I VmG1L E. B.uxu fl. 0 0 U) fl. < a- :::E CJ ...-'! ...J - rn 0 - UJ - ...-'! rn CJ ru .... ...-'! II) ...-'! a - CJ ~ ru ~ ""'I- II) 0 .... ==~ (') - 0 ~ ""'CJ \ ,i~l.. 118320 November 1982 BUREAU OF ECONO~nc CEO!"OCY THE UNIVERSITY OF TEXAS AT AUSTIN TO ACCOMPANY MAP-GEOLOGIC BUREAU OF ECONOMIC GEOLOGY QUADRANGLE MAP NO. 51 GEOLOGY OF THE HAMMETTS CROSSING QUADRANGLE, BLANCO, HAYS, AND TRAVIS COUNTIES, TEXAS Virgil E. Barnes 1982 CONTENTS General setting . 2 Cenozoic rocks . 4 Geologic formations . 2 Quaternary System . 4 Paleozoic rocks . 2 Pleistocene Series . 4 Pennsylvanian System . 2 Terrace deposit . 4 Marble Falls Limestone . 2 Recent Series · · · . 4 Smithwick Formation(?) . 2 Alluvium . 4 Mesozoic rocks . 3 Subsurface geology . 4 Cretaceous System (Lower Cretaceous) . 3 Mineral resources . 6 Trinity Group ........ \ . 3 Construction materials . 6 Travis Peak Formation . 3 Building stone . 6 Sycamore Sand . 3 Road material . 6 Cow Creek Beds . 3 Sand and gravel . 6 Shingle Hills Formation . 3 Water . 6 Hensen Sand Member . 3 References . 7 Glen Rose Limestone Member . 4 Appendix . 8 Bureau of Economic Geology, The University of Texas at Austin 2 GENERAL SETTING The geology of the Hammetts Crossing quad­ occupy the remainder of the quadrangle. The rangle is shown on a U. S. Geological Survey relatively flat-lying Cretaceous rocks dip eastward 7 .5-minute topographic quadrangle map and is the about 17 feet per mile. seventeenth geologic map in the Llano region to Hammetts Crossing quadrangle is mostly within appear on a modem 1:24,000 scale, 20-foot the Pedemales River basin. The river, in incised contour interval base. The relief in the quadrangle meanders, crosses the northern part of the quad­ is about 670 feet, elevations range from 681 feet, rangle from west to east. Tributaries draining into pool level of Lake Travis, to about 1,350 feet at a Pedernales River and the Lake Travis portion of point on the divide between Pedemales River basin the river, incised in their lower reaches, include and the Barton Creek drainage area. Flat Creek and its tributaries Calahan and Hammetts Crossing quadrangle, in the Hill Sycamore Creeks, Dead Mans Hole Creek, Roy Country southeast of the Llano region, is at the Creek, Hamilton Creek, Cypress Creek, and juncture of the Llano uplift and the Ouachita numerous unnamed drainages. A small area in the structural belt. Cretaceous rocks occupy the bulk southeastern part of the quadrangle is drained by of the quadrangle. Some alluvium, one terrace Barton Creek which enters Colorado River at deposit, and a very small area of Paleozoic rocks Austin. GEOLOGIC FORMATIONS PALEOZOIC ROCKS Feet Inches 22. Limestone, dark gray, massively bedded, nonfossiliferous; PENNSYLVANIAN SYSTEM thin chert layers on its upper surface 6 6 Marble Falls Limestone 21. Limestone, thin bedded, black, shaly, beds 1 /a to Plummer (1950, pp. 49-50) described 410 feet 3 inches thick. In some horizons the rock resembles of Marble Falls Limestone along Battle Branch and a tough, black shale more Pedemales River mostly in the Pedernales Falls than a limestone. The quadrangle (Barnes, in press). The upper few tens thicker limestone slabs of feet of this section are in the Hammetts Crossing have a tendency to break quadrangle. The upper three intervals of Plummer's into long, rectangular blocks having "fence section, reproduced as follows, will cover all of the post" shapes, resembling Marble Falls Limestone within the Hammetts some of the layers at the Crossing quadrangle and perhaps more. top of the section in the Cherokee Valley 20 6 Feet Inches Lower Cretaceous- Smithwick Formation(?) Travis Peak Formation- 24. Conglomerate, large, well-rounded Just downstream from the foot of Pedernales cobbles in a matrix of Falls, two exposures of weathered greenish-gray coarse, red sand partially cemented with calcium shale in seepy areas were present at the time the carbonate Pedemales Falls and Hammetts Crossing quad­ Pennsylvanian- rangles were mapped. These outcrops were covered Marble Falls Group- by a gravel bar the last time I was there (May 1, 23. Limestone, dark gray, mas· 1971), but the next great flood will probably sively bedded; layers 1 to 3 feet thick with prom· expose them again. One of these outcrops is on the inent partings, mostly Hammetts Crossing quadrangle and the other is on covered by talus 21 6 the Pedemales Falls quadrangle. Geology of the Hammetts Crossing Quadrangle, Blanco, Hays, and Travis Counties, Texas 3 Although these outcrops were not positively The Cow Creek Limestone is characteristically a identified as Smithwick, they are thought to be light-colored coquinite in which many of the shells Smithwick because of the presence of a normal have been dissolved to produce a honeycombed thickness of Marble Falls Limestone and because appearance. Pebbles, common in the Cow Creek to the shale is in the position that the Smithwick the west, are scarce to the east. The Cow Creek normally occupies. However, the exposures were so Limestone grades downward to the Hammett poor that one cannot be sure that the shale has not Shale; oysters are common in the zone of transi­ slumped from the upper valley wall where tion. The upper contact with the Hensell Sand is Hammett Shale crops out. sharp. The Hammett Shale and underlying Sycamore MESOZOIC ROCKS Sand are recessive because of weathering, causing the massive Cow Creek Limestone to form cliffs CRETACEOUS SYSTEM (LOWER CRETACEOUS) 1ining Pedemales River and its tributaries. Hamilton Creek plunges over a cliff of Cow Creek Trinity Group Limestone a distance of 50 feet or more to form Hamilton Pool, a popular swimming hole. The falls Travis Peak Formation has undercut the Cow Creek at least 100 feet. Another falls at Dead Mans Hole has a 30-foot or Sycamore Sand.-The Sycamore Sand consists of so drop. a basal conglomerate overlain in most places by strata composed of various proportions of sand, Shingle Hills Formation (Barnes, 1948) silt, clay, and pebbles. In the vicinity of Pedernales Falls, the Sycamore is limited to well-cemented Hensell Sand Member.-The Hensell Sand Mem­ conglomerate and its upper surface has been bored ber in the Hammetts Crossing quadrangle forms a by marine molluscs. To the east this bored surface bench above the Cow Creek Limestone; the bench is represented by sparsely distributed bored steepens near the Glen Rose contact. The Hensell is pebbles and cobbles mostly of Ellenburger car­ composed of conglomerate in places at its base bonate rocks. These pebbles mark the boundary followed by silty and argillaceous sand grading up between the reddish-colored sediments at the top to silt and clay, in part calcareous, in the upper of the Sycamore and the overlying grayish-green­ part. However, tongues of coarser grained material weathering Hammett Shale. The conglomerate is commonly occur at various levels. One indurated composed of rocks from almost the entire pre­ conglomerate bed about 25 feet above the base of Cretaceous stratigraphic column of Central Texas, the Hensen forms a bench in the western part of and some boulders, especially those of Hickory the quadrangle and, eastward along Flat Creek, Sandstone, are more than a foot in size. grades into massive limestone. Channel deposits of Cuyler (1939, fig. 1) shows about 97 feet of coarse sand and granules are present here and Sycamore Sand at Hammetts Crossing. At the same there; one is exposed along Ranch Road No. 962 point I measured 40 feet of Sycamore from river near the entrance to Hamilton Pool. level to the bored-pebble zone. This indicates that The lower part of the Hensen is mostly red, Cuyler included 57 feet of Hammett Shale with the other colors are common, and upward the reds give Sycamore Sand. way to various shades of gray, yellowish gray, and Cow Creek Beds.-Lozo and Stricklin (1956) greenish gray. Very little Hensell is cultivated subdivided the Cow Creek Beds into Cow Creek within the quadrangle; mostly it is used for Limestone above and Hammett Shale below. They rangeland. cored about 44 feet ' of Cow Creek Limestone and Hazzard and others (1949, pl. 2) measured 45 62 feet of Hammett Shale at Hamilton Pool near feet of Hensen Sand in the vicinity of Hamilton the eastern edge of the quadrangle. Pool. This measurement is somewhat thinner than The Hammett Shale is a sandy, silty, light the roughly 60 to 80 feet shown on the geologic grayish-green-weathering clay which is medium map of the Hammetts Crossing quadrangle. The gray when fresh. Westward in a shoreward direc­ boundaries of the Hensen were traced stereo­ tion, sand is more abundant and some conglom­ scopically on aerial photographs then transferred erate is present. Oysters were collected at locality by pantograph to the Hammetts Crossing 2-19A. quadrangle scribe sheet. The Hensell thickens 4 Bureau of Economic Geology, The University of Texas at Austin somewhat westward as bench-forming beds at the quadrangle and about 330 feet is present in the base of the Glen Rose die out westward. southeastern part. Cuyler ( 1939, p. 637) mentions finding The Glen Rose is mostly suited to ranching. In Orbitolina texana Roemer in the Hensell along general, vegetation on the Glen Rose is sparser than Pedemales River a few miles northeast of Hamilton on other units, indicating the relative sterility of its Pool. Orbitolina also occurs in the transitional zone soil. from Hensell to Glen Rose in the Spicewood The Salenia texana zone also contains abundant quadrangle (Barnes, in press).