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Lab 6 - Cretaceous Field Trip

Lab 6 - Cretaceous Field Trip - LAB 6 FOSSILS GEOLOGY AND...

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LAB # 6 - FOSSILS, GEOLOGY AND DEPOSITIONAL ENVIRONMENTS OF THE CRETACEOUS SYSTEM IN NORTH-CENTRAL TEXAS The Comanche Series (Lower Cretaceous) is one of the two primary subdivisions of the Cretaceous in North America. In his travels through the Republic of Texas in the 1840's, the German geologist Ferdinand Roemer correctly determined the age of these rocks as Cretaceous. A number of years later, Robert T. Hill discovered thick beds of fossil oysters in the same region. He would later name this series of rocks the Comanchean, after his home town of Comanche, where he first found the oyster beds. At the same time he named the series of rocks representing the Upper Cretaceous in North America, the Gulfian Series, based on rocks he studied along the Gulf Coastal Plain of Texas. The Comanche Series covers about the first half of Cretaceous history in North America. The outcrop area of the Comanchean is tremendous, covering a broad, three hundred mile-long strip from the north-central part of the state to southwest Texas. The environment of deposition indicated by these outcrops probably represents terrestrial, nearshore and lagoonal facies. In addition, subsurface data indicates that basin and barrier reef facies were also present. In all, there appear to have been six reef-building episodes during Comancean time and after each cycle an influx of clastic sediments wiped out the reef-building organisms. Most of these reefs were composed of rudists, strange coral-like bivalves with an elongate, tubular valve whose base was cemented to the ocean floor. These barrier reefs trended along a line that roughly paralled the modern Gulf Coast. Behind the reef, in an area within the central part of the state, sediments were being deposited in a lagoon and/or upon a carbonate platform. Throughout the Comanchean of Texas, there are a number of sand-limestone cycles representing terrestrial and shallow marine facies respectively. Each couplet also indicates a transgressive (submergence of land) and regressive (emergence of land) phase. In the Lower Cretaceous of north-central Texas there were three major transgressive/regressive cycles. We will be studying two of them. The lowermost transgressive/regressive couplet is represented by the Glen Rose Limestone and Bluff Dale Sand (Twin Mountains Formation), which we will study in detail later during the field trip. The first stops, within the Paluxy, Walnut, Comanche Peak and Edwards Formations, represent the second marine transgression during the middle portion of Comanchean times. STOP #1 - Overview of the Fredericksburg Group. This is a view of the topographic features east of the intersection of Highways 67 and 2481 on Hwy 67. The rocks belonging to the formations of the Trinity and Fredericksburg Groups outcropping in central Texas directly influence the topography of the region.
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