F. Jerry Lucia, 2012. "The Great Lower Ordovician Cavern System", Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia, James Derby, Richard Fritz, Susan Longacre, William Morgan, Charles Sternbach
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Karsting and collapse brecciation in the Lower Ordovician carbonates have been recognized for many years. However, the time of cavern formation and the geochemical hydrology responsible is debatable. In this chapter, I intend to review pertinent literature and evaluate the evidence of the presence of paleocaverns, the time of their formation, the history of cavern collapse to form collapse breccias, and the relationship of collapse breccias to structure. I will not review chemical hydrology issues because a discussion on the geochemical environment is pertinent only after the time of cavern development has been adequately resolved. The most robust data sets come from outcrop studies. Outcrops with extensive exposures reviewed here are the El Paso Group in the Franklin Mountains, west Texas; the Pogonip Group in Nopah Range, southeastern California; and the St. George Group in Newfoundland. The Lower Or-dovician outcrops in central Texas, the Mississippi Valley, Virginia, and the Arbuckle Mountains are also useful. Robust subsurface data sets include the Ellenburger Group of Texas; the Knox Group of Tennessee, Kentucky, and Ohio; and the Arbuckle Group of central Kansas. Core descriptions from the subsurface Arbuckle Group in Oklahoma and Arkansas are also helpful.
The most convincing evidence of cavern formation is roof collapse, that is, evidence that breccia blocks are below their stratigraphic positions. The time of cavern formation is more difficult to ascertain and most commonly is based on the source of the infilling sediment by comparing lithologies and, in places, using biostratigraphy. The history of the collapse can be determined only in the most extensive outcrops and mining operations, although modern three-dimensional (3-D) seismic volumes are useful. The relationship of collapse breccias to structure is basically a timing issue and can be resolved only by detailed geologic studies.
I conclude from reviewing published data that convincing evidence shows that an extensive cavern system existed in the Lower Ordovician carbonates at the time of the Sauk-Tippecanoe unconformity. In some areas, the unconformity surface is highly irregular and appears to represent karst terrain. Caverns located far below the unconformity were most likely formed in response to internal disconformities. Lower Ordovician fractures and faults can have a controlling influence on the location and geometry of the caverns. Collapse of these caverns produced the collapse breccia and fracturing of the cavern roof. In some instances, cavern collapse has produced structural sags similar to those produced by the expansion related to strike-slip faulting. In extreme cases, collapse of large caverns produced breccia pipes that extended more than 330 m (>1000 ft) into overlying Ordovician, Silurian, and possibly Devonian units.
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Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia
The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.