General

Rocks of the Packsaddle Schist metasediments are exposed in a road cut on the northeast side of highway 71 northwest of the Honey Creek bridge. These thinly interlayed gray, tan, black and light brown graphitic and quartzose phyllites, biotite schist and fine-grained quartzite are characteristic of the metasedimentary package of rocks that marks the base of the Packsaddle Domain. The age is uncertain, but felsic metavolcanic rocks elsewhere interlayered with this sequence crystallized about 1245 million years ago. A virtual field trip of the Packsaddle Schist is available at: http://www.lgsweb.org/field05c.html.

North of this locality (about 1/2 mile) along Honey Creek, the Packsaddle Domain metasediments are thrust over Valley Spring augen gneiss that has an age of 1270 million years ago.

There is a well-developed south-dipping cleavage in the outcrop. Although the outcrop looks little deformed, tight to isoclinal folds of the phyllites show highly repeated folding. The folds plunge 280 toward S-30-E.

A small granitic pegmatite intrudes across and distorts the cleavage of the graphitic phyllite. Granite plutonism post-dates cleavage formation. A thin bleached zone at the margins of the pegmatite indicates volatilization of graphite during intrusion.

Back around the turn of the century a newspaper article was published in San Antonio which was typical of the many legends of lost mines in the area of Llano and San Saba Counties. According to the article D.F. Brooks, "says he has discovered on Riley Mountain, west of the Packsaddle group in Llano County, an old Mexican or Mormon gold mine, and the remains of an old smelter a half mile west of Honey Creek and three miles west of the Packsaddle Mountains. The ledge, he says, is a true fissure, forty-five or fifty feet deep. The mine has a six-foot entrance, and both the foot and hanging walls are "pockety". In the center an eighteen-inch pay streak of decomposed sugar quartz is yellow as gold itself.

The Hickory Sandstone (Cambrian) is deep red, because of high iron content in abundant hematite. The sand grains are coarse and well rounded. There are also hematitic ooids, which are concentrically laminated sand-size grains that result from mineral precipitation on fragments moving about in agitated waters of the tidal zone. Tidal conditions are also indicated by cross-bedding, formed as sediment moved by water currents accumulated on a rippled bottom. Fossils of the inarticulate brachiopod Lingula are in hash zones within the sandstone. Trace fossil burrows can be observed on the bases of some of the sandstone layers.

The Hickory Creek Sandstone near Honey Creek rests in a fault bounded block surrounded by the Precambrian Town Mountain Granite and Valley Spring Gneiss. Small normal faults are exposed along the outcrop in the road cut. These faults offset the strata in the Hickory Sandstone from just a few centimeters to more than several meters. The offset along the faults is clearly visible in several cases by an obvious color change.

Road Log

legend for the map

mile 0.0 (30^o 45.590; 98^o 40.510) - Junction of 29, 16, 71

mile 1.4 (30^o 44.289; 98^o 40.594) - highway 71 departs to left (east-southeast)

mile 2.7 (30^o 43.947; 98^o 39.420) - preCambrian Spring Valley Gneiss outcrop on right side of highway. Small granite dikes are common at this location.

mile 4.0 (30^o 43.155; 98^o 38.454) - preCambrian Honey Fm. outcrop of bedded rock on right

mile 4.6 (30^o 42.894; 98^o 37.842) - preCambrian Honey Fm. outcrop of bedded rock on left side.

mile 5.4 (30^o 42.715; 98^o 36.641) - preCambrian Honey Fm. outcrop left side.

mile 6.7 (30^o 42.518; 98^o 36.000) - outcrop of Hickory Sandstone member of the Riley Formation (Middle Cambrian) on left; bedded sandstone.

mile 8.1 (30^o 42.303; 98^o 34.633) - outcrop of the Hickory Sandstone on left.

mile 8.8 (30^o 41.925; 98^o 34.027) - outcrop of Spring Valley Gneiss (preCambrian) on right. This outcrop is also shown in a closeup view.

mile 9.9 (30^o ; 98^o ) - highway 307 to Kingsland low-water crossing of the Llano River (see log under Shorter Highway Excursions)

mile 10.0 (30^o 41.135; 98^o 33.594) - Hickory Sandstone on left.

mile 10.4 (30^o ; 98^o ) - outcrop both sides, bedded Hickory sandstone. Thinner bedding is seen in the closeup view.

mile 14.3 (30^o 37.786; 98^o 31.733) - Honey Creek, Packsaddle Schist metasediments near base of the Packsaddle Domain; road cut on NE side of highway immediately NW of Honey Creek bridge.

A virtual field trip of this location can be seen at the 2005 Field Trip, Lafayette Geological Society field trip.

mile 14.9 (30^o 37.293; 98^o 31.474) - basal Cambrian Hickory Member of the Riley Formation is exposed in the ridge to the left. The Hickory is coarse to medium grained arkose and quartz arenite. contact between Hickory Sandstone and Packsaddle Schist is approximately at the break in slope. [18]

mile 15.8 (30^o 36.674; 98^o 31.089) - historical marker on left; view northeast of Riley Formation in the Packsaddle Mountain.

mile 16.5 (30^o 36.052; 98^o 30.704) - marbles of the basal Packsaddle Domain are locally exposed.

mile 20.1 (30^o 33.571; 98^o 28.477) - outcrops left and right; bedded schist (formation ?) with marble veins across bedding.

legend for the map

legend for the map

mile (30^o ; 98^o ) - highway xx to Kingsland

mile 20.2 (30^o 33.489; 98^o 28.377) - Sandy Creek. The sands are derived from the metamorphic rocks.

mile 22.3 (30^o 32.208; 98^o 26.580) - highway 962 on right to Round Mountain

mile 22.4 (30^o 32.347; 98^o 26.773) - outcrop (formation ?) on left.

mile 22.7 (30^o 32.103; 98^o 26.456) - outcrop (formation ?) on left.

mile 24.6 (30^o 31.284; 98^o 24.800) - outcrop (formation ?) on left.

mile 25.2 (30^o 31.037; 98^o 24.305) - Roadcut exposes Packsaddle Schist metasediments and metavolcanics in the lower Packsaddle Domain. The schist is cut with igneous intrusions; unpaved private road to left.

Marble Falls Fault juxtaposes the down-faulted Gorman Fm. against the preCambrian Packsaddle Schist (to the northeast. At Marble Falls, this fault has 3,000 feet of displacement.

mile 25.3 (30^o 30.997; 98^o 24.174) - Slick Rock Creek bridge. Fault brecciated Gorman Fm. dolostone is exposed in the stream channel ; probably related to Marble Falls fault.

mile 25.5 (30^o 30.939; 98^o 23.930) - roadcuts (formation ?) right & left, limestone.

mile (30^o ; 98^o ) - highway xx to Horseshoe Bay.

mile 26.4 (30^o ; 98^o ) - Honeycut Fm. shows paleokarst features incluiding large-scale solution conduits and terra rossa residual soils.

mile 27.5 (30^o 30.824; 98^o 22.033) - roadside park on left. exposure of medium bedded Gorman Fm.on right side, inside fence. [take photo]

mile 28.0 (30^o 30.922; 98^o 21.531) - outcrop (formation ?) on left, limestone.

mile 28.8 (30^o 31.040; 98^o 20.748) - road cut (formation ?) right & left, limestone.

mile 29.9 (30^o 30.915; 98^o 19.549) - outcrop (formation ?) on left, white limestone; but the outcrop on the right side of the highway is different (formation ?).

mile 30.9 (30^o 30.756; 98^o 18.560) - outcrops of Ellenburger Group (Ordovician Gorman Formation) on both sides of road. The Gorman Fm. is a cherty interbedded limestone and dolomite overlying the Honeycut Fm.

mile 31.8 (30^o 30.551; 98^o 17.763) - intersection with U.S. 281; turn left (north) for Marble Falls

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