FIELD TRIP STOP – A CRETACEOUS OFFSHORE BAR IN CENTRAL TEXAS
LOCATION: The abandoned Whitestone Quarry just north of Road 1431 near the Williamson – Travis County line, three miles west of Whitestone.
GEOLOGIC FEATURES: Offshore Bar; Shoreface; Oolites; Crossbedding; Regression
DESCRIPTION: The Whitestone Lentil Member of the Walnut Formation represents an offshore bar, about a mile wide, that paralleled a Lower Cretaceous lagoon and shoreline to the northeast and an oceanward side to the southwest. This lentil is located stratigraphically between the Cedar Park Limestone Member below and the Keys Valley Marl Member above.
The Whitestone Lentil was quarried at this location. It is known commercially as the Cordova Limestone. This, in turn, is subdivided into two subdivisions – the Cordova Cream and the Cordova Shell. The Cordova Cream (also called the Upper Oolitic Facies) is nearly fossil free and is an oolitic limestone. The Cordova Shell (also called the Lower Trigonia Facies) contains many molds of the clam Trigonia and other fossils such as the snail Turritella. The Upper Oolite Facies is thought to represent a higher energy Upper Shoreface environment and the Lower Trigonia Facies represents a lower energy Lower Shoreface.
Rocks in the Upper Shoreface (Upper Oolitic Facies) have sedimentary structures indicative of a subtidal high-energy environment, such as oolites and crossbedding that are readily affected by wave and current action. Rocks in the Lower Shoreface (Lower Trigonia Facies) had enough energy to linearly orient elongate Turritella shells and flip clam shells to a stable convex-up position, but not strong enough to size sort the sediment.
The Oolitic Facies is found overlying the Trigonia Facies due to seaward accretion of sediment. Lagoonal sediments containing oysters of the low energy Keys Valley Marl are found over the lentil due to continues regression of the shoreline.
(1) How are oolites formed and in what environment are they associated?
(2) Where are oolites forming today?
(3) Large-scale crossbedding can be found in sand dunes. In what types of environments are small-scale crossbeds found?
(4) What is a Shoreface and what is its relation to wave base?
(5) CHALLENGE: What are Transgressions and Regressions and how can they be identified in the geological record?
(6) CHALLENGE: What physical processes are responsible for the formation of Offshore Bars?
-Merrill, Glen K. 1980. Road Guide to the Geology of the Llano Region, Central Texas. Guidebook to the Annual Field Trip of the East Texas Geological Society. Oct. 19-21, 1980. Publication No. 80-73, pp. 198-199
Figure 1 - The abandoned Whitestone Quarry where the Whitestone Lentil Member of the Lower Cretaceous Walnut Formation is exposed.
Figure 2 - The Whitestone Lentil Member is composed of two parts - the Lower Trigonia Facies and the overlying Upper Oolitic Facies. These rocks are widely used around the world as a building stone.
Figure 3 - Rocks exposed in the quarry show the effects of weathering that disguise details. Here we can see the interbedded layers of two facies - the crossbedding present in the Oolitic Facies and the shelly Trigonia Facies.
Figure 4 - A bedding plane view of the Upper Trigonia Facies.
Figure 5 - A building stone that has been cut parallel to a bedding plane of the Lower Trigonia Facies.
Figure 6 - A close up of Figure 5.
Figure 7 - A building stone cut parallel to the bedding surface of the Trigonia Facies.
Figure 8 - Pelecypods other than Trigonia are found in the fossiliferous facies. In addition, note the presence of snails such as Turritella and the variety of particle sizes.
Figure 9 - Trigonia and Turritella commonly occur together.
Figure 10 - A Rudistid clam is seen in the center of the photo.
Figure 11 - High-spired Turritella specimens are common in the Trigonia Facies.
Figure 12 - In places, Turritella shells appear to be oriented in a particular direction, commonly with their apices pointing in a similar direction,
Figure 13 - A vertical section of the Whitestone Lentil illstrating the interfingering of the Upper Oolite Facies having crossbedding and the shelly Lower Trigonia Facies. Since this is a photo of a building stone, it is assumed that the Trigonia shells were deposited in a convex-up orientation since this is the way that these shells are usually found in outcrop.
Figure 14 - Crossbedding of oolites in the Upper Shoreface environment of the Whitestone Lentil.