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PEDERNALES FALLS
FIELD TRIP STOP -- PEDERNALES FALLS STATE PARK, TEXAS (Pennsylvanian Inclined Beds with Cretaceous Angular Unconformity)
LOCATION: From Houston, TX proceed west on I-10 until you reach Columbus [75 miles]. Go northwest on Highway 71 towards Austin [86 miles]. In Austin take 290 West until you reach 3232 [32 miles]. Turn right (north) and follow the signs for Pedernales State Park [6 miles].
GEOLOGIC FEATURES: Pennsylvanian Limestone; Cretaceous Conglomerate, Tilted Sedimentary Beds; Horizontal Beds; Angular Unconformity; Potholes; Fossils
DESCRIPTION: At this site, you will park and walk down to the Pedernales River. Notice that the strata in the river bed are Pennsylvanian Limestones (Marble Falls Limestone – about 300 myo) that are tilted to the east from their original horizontal position. Fossils of crinoid stems are scattered throughout. Limestones are commonly pitted from chemical weathering. The Lower Cretaceous and horizontal Sycamore Conglomerate (about 120 myo) overlies the limestone. It is composed of rounded pebbles and cobbles. The conglomerate is located at the top of the bluff and can sometimes be partially obscured by vegetation. Also found at this site are discontinuous calcite-filled veins, and patches of chert, a form of microcrystalline quartz
STUDENT QUESTIONS:
(1) What causes the pools of water to accumulate?
(2) Why are the Pennsylvanian rocks tilted?
(3) Why do the fossil crinoids appear as disks?
(4) What exactly is an Unconformity?
(5) Describe the geologic events responsible for the formation of the Angular Unconformity.
(6) How much time is missing at the Unconformity?
(7) What is a possible cause of the Conglomerate?
(8) Which Geologic Periods are missing?
(9) How might the potholes been formed in the limestone?
SELECTED REFERENCES:
-Wermund, E.G. and V.E. Barnes. 2003. Down to Earth at Pedernales Falls State Park, Texas. Bureau of Economic Geology. The University of Texas at Austin, Austin, Texas. 48 pp.
PHOTOS:
Figure 1 - Notice the tilted beds of limestone found in the river bed as well the bluff. The river flows to the right (east) along the top of the bedding forming a pool due to the position of the overlying bed.
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Figure 2 - The superposition of inclined strata allows for the formation of ponds along the length of the river.
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Figure 3 - Ponding due to inclined nature of the strata. The bluff on the far side of the river is composed of the horizontally bedded Sycamore Conglomerate (Lower Cretaceous).
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Figure 4 - Looking towards the northeast are inclined limestone beds overlain by horizontal conglomerates in the bluff.
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Figure 5 - Inclined surfaces of the Marble Falls Limestone are irregular and pitted due to dissolution. Horizontal beds on the bluff represent the Lower Cretaceous Sycamore Conglomerate which lies in Angular Unconformity on the Pennsylvanian Marble Falls Limestone.
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Figure 6 - Joints in the limestone can be eroded into cracks often accompanied by semicircular potholes formed by swirling eddies carrying abrasive particles of rock.
Figure 7 - Fossil fragments of crinoids (columnals and stems formed of columnals) are commonly dispersed within the Marble Falls Limestone.