This page contains information on the early Eocene Hannold Hill Formation, a fluvial sandstone and conglomerate sequence exposed in the Big Bend region of West Texas. The early Tertiary strata in this region accumulated during the Laramide Orogeny in the Tornillo Basin (Lehman, 1991). The Hannold Hill Formation is thought to have been deposited during the main pulse of Laramide tectonism.
The Hannold Hill Formation comprises a series of fluvial sandstones, conglomerates, and mudstones of early Eocene age exposed in Big Bend National Park, Brewster County, Texas. The Hannold Hill Formation consists of four stratigraphic units: the Exhibit Ridge Sandstone Member, the lower mudstone member, the upper sandstone member, and the upper mudstone member. The lower boundary of the Hannold Hill Formation is raised to the base of the Exhibit Ridge Sandstone. A new type section should be formally established for the Hannold Hill Formation because the present type section includes strata from the underlying Black Peaks Formation. Sediments of the Hannold Hill Formation were derived from foreland uplifts exposed during the Laramide Orogeny. Channel lag gravels and sandstones were derived primarily from erosion of Cretaceous limestones and shales exposed to the west and northwest of the Tornillo Basin. Conglomerate from the upper sandstone member is coarser than conglomerate from the Exhibit Ridge Sandstone Member; however, pebble lithologies and sandstone compositions are the same for the Exhibit Ridge Sandstone and upper sandstone members. Hence, the sediment source terrain remained the same during deposition of the Hannold Hill Formation. Paleocurrent data for both channel sandstones show a relatively wide variation in direction. The current direction was predominately toward the southeast and east during deposition of Hannold Hill sandstones. Meandering streams deposited the sands and muds of the Hannold Hill Formation.
All of the known exposures of the Hannold Hill Formation lie within the central part of the valley of Tornillo Creek, one of the major ephemeral tributaries of the Rio Grande in Big Bend National Park. The broad valley of Tornillo Creek is refered to as "Tornillo Flat" in this area. The Hannold Hill Formation was named for Hannold Hill located in south-central Tornillo Flat (Maxwell et al., 1967).
The Exhibit Ridge Sandstone of the Hannold Hill Formation outcrops at the Fossil Bone Exhibit. It is the basal sandstone unit of the formation. Pebble conglomerate at the base of the unit marks the contact with the Black Peaks Formation. The conglomerate contains limestone, black chert, sandstone and petrified wood. The upper part of the sandstone body is trough cross bedded.
The Exhibit Ridge Sandstone of the Hannold Hill Formation is the lowermost sandstone and outcrops at this location. Measured thickness range from 1.7 m to 10 m. The average is about 6 m based on eight measured sections.
Exhibit Ridge extends northwest from the fossil bone exhibit in the Tornillo Flat area of the Park. Outcrops of the Hannold Hill Formation along Exhibit Ridge are about 25 m thick.
The mudstone unit beneath the Exhibit Ridge Sandstone varies in thickness, and is not easily mapped. However, the Sandstone is an easily mappable lithostratigraphic unit. My thesis suggested that Exhibit Ridge would be better type section than the rock crusher locality (established by Maxwell et al. 1967) in eastern Tornillo Flat.
Field Sketch showing relationships observed between the Hannold Hill Formation and the Canoe Formation 0.7 km west of U.S. 385 (N 65 W from
BM 2803 of Figure 21). Units are: 1) upper sandstone member, 1a) thin gray-brown sandstone just above upper sandstone member, 2) gray mudstone, 2a) orange mudstone, 3) lignite bed, 4) gray mudstone with yellow petrified logs and stumps encrusted with gypsum and limonite, 4a)brown sandstone concretions, 5) purple and gray banded mudstone, 6) Big Yellow Sandstone Member of the Canoe Formation, 7) gray mudstone and sandstone with red-brown concretions. Approximate height of outcrop is 15 meters.
The Hannold Hill Formation is exposed northeast of the Grapevine Hills. Hannold Hill outcrops in the North Grapvine Hills (NGH) area range from 19 m to 33 m thick.
East of the Grapevine Hills, the Hannold Hill Formation pinches out and only the upper mudstone is exposed. At the type section (Rock Crusher locality) only the upper mudstone unit is present below the Big Yellow Sandstone.
Photograph of the Exhibit Ridge Sandstone member tilted beneath horizontal beds of the upper sandstone member in the NGH area.
Rutile crystal inclusion in quartz grain. Photo taken in cross polarized light. Plutonic quartz is the most abundant quartz type in the Hannold Hill sandstones.
Reworked fragment of a Cretaceous inoceramid bivalve in Hannold Hill sandstone sample ER1-1. Magnification is 10 X, width of the field of view is 1.0 mm.
Sedimentary rock fragment (shale) in Hannold Hill sandstone sample CV2-1. Magnification is 20 x, width of the field of view is 0.5 mm.
Created on Mar 28, 1999
Heather Beatty's Home Page
hlbeatty@worldnet.att.net