Oklahoma Geological Survey Observatory near Leonard, OK
Visiting the 300 million year old reef-Pennsylvanian/ Upper
Carboniferous outcrops at Bixhoma Lake Park near Leonard, OK
The photo shows students searching shale beside Bixhoma Lake for fossils. The area has fewer fossils than are found in many Pennsylvanian age rocks of Northeast Oklahoma. Still, there are enough fossils of brachiopods, crinoid stem fragments, and coral to show that this was once a coral reef in a shallow tropical sea. As the sea grew shallower, sediments, from the Ouachita Mountains to the south, begin to deposit sand, seen in a thin sandstone band (just above the head of the young man in the red shirt). Later the the sea deepned and the shoreline moved far enough away that only silt was deposited, producing a gray shale. Finally the sea became very shallow and the massive sandstone at the top of the photo began to deposit. More cycles of shallow sea transgression-regression (sea moving in and out) are seen in the park. The entire area is a deep valley cut through soft shales and thin sandstones by Mountain Creek. The surrounding hills (or cuestas) are preserved by a thick sandstone which resists erosion, although it slowly breaks off in huge blocks (some 10 feet along one side) at its edges. That thick resistant sandstone is perhaps 80 feet above the Lake. The section in the photo is about 20 feet above the lake. According to A. P. Bennison, on pages 14 to 31 of _Tulsa's_Physical_ Environment_ published in 1972 by the Tulsa Geological Society, the rocks seen are around the lake are Pennsylvanian System, (outside the USA this is the upper part of the Carboniferous System), Desmoinesian Series, Marmaton Group, Wewoka Formation. The rocks were deposited from 320 to 280 million years before the present, and each change (eg from near shore to far shore, may have averaged 88,000 years. A very detailed core log includes all the rocks outcropping in the park. The mini field trip to the 300 million year reef is directed toward younger students who want to spend a total of several hours on geophysics/geology in one day. OGS staff at Leonard includes a geophysicist who can guide younger persons. A geology class may require a more expert guide. In that case, their leader should prepare herself (or himself) by reading Bennison (cited above) and visiting the park in advance. For aid in such advance preparation contact Jim at 918-366-4152, or jim@okgeosurvey1.gov.
