THE LOWER ORDOVICIAN MASCOT FORMATION, UPPER KNOX GROUP, IN NORTH CENTRAL TENNESSEE. PART I: PALEOENVIRONMENTAL HISTORY. PART II: DOLOMITIZATION AND PALEOHYDRAULIC HISTORY

[摘要] Part I. Petrographic examination of Lower Ordovician limestone and dolomite fabrics indicates that the sedimentary column in central Tennessee reflects multiple seaward progradation episodes of peritidal sediments across a broad, shallow shelf. The following sedimentary types were recognized: columnar, spheroidal, thrombolitic, domed, laminar and crinkled algal sediments; intramicrites, intrasparites, pelmicrites, laminated micrites rich in clay and silt, algal-bound biolithites, oosparites with either tangential or smaller radial ooids, oomicrites and orthoquartzites. Vertical distribution of these sediment types within the stratigraphic column indicates that normal marine, transgressive waters were gradually displaced by progressively more saline waters originating from prograding evaporative tidal flats and restricted lagoons. The waning phases of such episodes are commonly marked by sheet-like deposits of coarse, unimodal sands derived from the distant craton. Because these sheets were deposited and reworked on tidal flat sediments during periods of maximum subaerial exposure, they are reliable lithostratigraphic markers. An isopach map of the upper Mascot Formation and additional evidence of progressive shallowing throughout the depositional area suggest that the Nashville Dome may have become a positive feature during the early Ordovician. Continued uplift coincided with the development of the Sauk unconformity. Part II. Mascot Formation dolomite is made up of several genetically distinct dolomite types that are recognized by their combined petrographic, cathodoluminescent, and geochemical characteristics. Cathodoluminescence, however, is the key investigative tool used to distinguish and classify dolomite types into 2 basic categories and 5 subcategories: NZh, and NZc dolomite are subcategories of NZ dolomite rhombs exhibiting Non Zoned luminescence; Z(,1), Z(,2), and Z(,3) dolomite are subcategories of Z dolomite exhibiting Zoned luminescence and three or more concentric luminescent zones. Based on macrotextural and petrographic relationships observed among dolomite categories, most NZh, NZc and, Z(,1) dolomite grew penecontemporaneously on evaporative tidal flats and following deposition of prograding peritidal sediments, within shallow, schizohaline, subsurface sediment environments. Recognizable recurrent sequences of abundant luminescent zones occurring in Z(,2) dolomite can be correlated in subsurface samples throughout a geographic area exceeding 1300 km(;;2) and a stratigraphic thickness exceeding 250 meters. Additional evidence obtained using an established microstratigraphic sequence of these luminescent zones records the southward movement of regional dolomitizing solutions both laterally and upward through karsted carbonate sediments. These, other petrographic, and geochemical data, indicate that regional dolomitization occurred within subsurface marine-meteoric water mixing zones that formed during the marine inundation of an incipient, subareally exposed, Nashville dome. As southward-transgressing middle Ordovician seas drowned topographically high meteoric recharge zones, dolomitization ceased. Following at least one additional karsting episode, Mississippi Valley-type mineralizing solutions invaded the study area. These transported zinc, iron, and sufficient magnesium to precipitate Z(,3) dolomite before and immediately following sphalerite precipitation. The common occurrence of multiple dolomite overgrowths exhibiting several luminescent characteristics, and other petrographic evidence, indicate that maximum permeability pathways for subsurface fluids were generated during the earliest karsting episode following collapse of multiple, thin and permeable grainstone horizons. These pathways channeled subsurface fluid flow from the end of early Ordovician time to at least Mississippian and possibly Pennsylvanian time.

[发布机构] Rice University