AN OVERVIEW OF THE SEISMIC CHARACTERISTICS OF LAYERED SEQUENCES OF THE PRECAMBRIAN BASEMENT OF THE SOUTHERN MID-CONTINENT

Seismic reflection data that image buried Precambrian rocks of the southern mid-continent commonly exhibit layering that recent work suggests has a variety of different origins. Layering beneath the Hardeman Basin of Oklahoma and Texas, first imaged in COCORP data in the 1970’s, is characterized by two or three very strong reflection packages against which lower amplitude reflections truncate in downlap or onlap geometries. Packages of strong reflections are also found beneath the Palo Duro Basin of the Texas Panhandle. Data from this region shows downlap and onlap against the strong reflectors as well as crosscutting relationships among the strong reflectors. Thus, while data from the Hardeman Basin might favor a sedimentary origin, data from the Palo Duro Basin are suggestive of an igneous origin. Drill hole data, and recent geochemical and geochronologic data point to an episode of bimodal igneous activity at ~ 1.3 Ga, that likely included intrusion of mafic sills and deposition of volcanoclastic strata, as the origin of the reflectivity. In the central Permian basin, along the Texas-New Mexico border, and in the Tucumcari basin of New Mexico, basement reflectivity is due to extensional activity at 1.2 to 1.1 Ga. Seismic data from the central Permian Basin exhibits pervasive layering over a large area, that is clearly correlated to a ~1.1 Ga massive layer intrusion, termed the Pecos Mafic Intrusive Complex (PMIC), on the basis of a synthetic seismogram from the North American Royalties #1 Nellie well which penetrated 4 km of the intrusion. Further north in the Tucumcari basin, seismic data show that the metasediments and the metavolcanics of the ~1.2 Ga Debaca-Swisher terrane unconformably overlie basement that is probably comprised of ~1.3 Ga felsic rocks. Well data suggest that strong sub-horizontal reflectors in this package are due to mafic sills. New geochemical and geochronologic data from wells that penetrate basement in eastern New Mexico and the Texas Panhandle has been a major contributor to a more detailed understanding of the origin of reflectors in the southern mid-continent.