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THE STRATAGRAPHIC LOCATIONS OF INOCERAMUS, CLADOCERAMUS, UNDULATOPLICATUS IN THE BIG BEND AREA OF TEXAS
Most other geologists have mistakenly assumed IUP was an index fossil for the top of the Boquillas where they are found on the southeast side of the Big Bend. Some have even mistakenly reported IUP to be in the Boquillas Formation on the west side of the Big Bend area when in fact they only found flat inoceramids there. IUP are only found in the overlying Pen Formation, clay, on the west side of the Big Bend area.
The locations of IUP on the Southeast side of Big Bend National Park, (BBNP), are easily located in the top of the Boquillas Formation near or at the contact with the overlying Pen Formation. Several areas were studied. No IUP were found in the overlying Pen Formation in this area. However > 30 km northwest of these areas, in the Cottonwood creek area, IUP are found +/- 30 m above the base of the overlying Pen Formation. In the Terlingua Ghost Town area, another >20 km west of the Cottonwood creek area, IUP are found +/- 30 m above the base of the overlying Pen Formation. There are three other locations, in the Studdy Butte and the Terlingua creek area, where IUP are located in the Pen formation, but no section could be measured because of faulting and or the top of the Boquillas Formation was not exposed in the area.
These findings are important for dating (Ma) marine fossils found in this area such as mosasaurs, etc. Other paleontologists assumed that the mosasaurs that we discovered in the western Big Bend area were only +/- 83 mya because they concluded IUP should be near the same level as the sites, not 60 m above the sites. Now we believe the mosasaurs we discovered on the west side are more like 87 + Ma. The Western Interior Seaway was receding to the southeast at this time and its rate of recession can now be better estimated.