GEOG 1710: Rocks and minerals of North Texas and Southern Oklahoma.
Introduction & Background.
Outcrops of sedimentary and igneous rocks in North-Central Texas and
Southern Oklahoma can be visited on this virtual field trip. Rocks
underlying the DFW region formed in the Cretaceous Period, about 100 million years
ago.
Fig. 1. Cretaceous paleogeography of the U.S.
North-Central Texas was under a shallow subtropical
ocean during most of this period. Sand was deposited in coastal areas (beaches,
river deltas), mud and clay accumulated offshore from river mouths and limestone
formed further offshore. Layers of sandstone, shale (mud and clay), marl
(calcareous shale)
and limestone formed from these sediments and were later tilted and eroded, such
that their outcrops have a north-south strike and a gentle dip to the east.
Fig. 2. Geology of Dallas County. Numbers refer to field trip sites.
Four major outcrops run through Dallas County:
Woodbine Sandstone on the Tarrant County border.
Eagle Ford Shale in western Dallas County.
Austin Chalk in central Dallas County.
Taylor Marl in eastern Dallas County.
(river sediments, shown in yellow and orange on the map, are much more recent than the Cretaceous bedrock).
Unlike North Texas, Southern Oklahoma
was severely deformed by tectonic movements that occurred in the Pennsylvanian
Period (about 300 million years ago). Numerous faults and folds were created and
some areas were uplifted and then eroded, forming hills (from the uplift) and
sedimentary rocks (from the erosion). These features resulted in the Arbuckle
Mountains of Southern Oklahoma.
Fig. 3. The Arbuckle Mountains of Southern Oklahoma. Numbers
refer to field trip sites. Colors represent different rock outcrops.
The Arbuckle Mountains region is heavily faulted (black lines on map).
Rock outcrops to be visited on the virtual field trip are:
Collings Ranch Conglomerate near Davis (stop 5 on map).
Colbert Rhyolite near Davis (stop 6 on map).
Tishomingo Granite near Troy (stop 7 on map).
Virtual Field Trip.
1. Taylor Marl in eastern Dallas
County.
2. Austin Chalk in central Dallas County.
3. Eagle Ford Shale in western Dallas County.
4. Woodbine Sandstone on the Tarrant County border.
5. Collings Ranch Conglomerate
near Davis.
6. Colbert Rhyolite near Davis.
7. Tishomingo Granite near Troy.
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