TIMING AND NATURE OF FLUID ALTERATION WITHIN THE UPPERMOST ELLENBURGER GROUP CARBONATES, FORT WORTH BASIN, TEXAS: A PALEOMAGNETIC AND DIAGENETIC STUDY

Petrographic, geochemical and paleomagnetic data were collected from the uppermost portion of the Ellenburger Group carbonates (Ordovician) in oriented well core in Fort Worth Basin, Texas, to test whether externally-derived fluids from late Paleozoic Ouachita orogenesis altered the unit during development of the basin. Samples were taken from lithofacies ranging from limestones to heavily brecciated and/or veined dolomites. Vein and vug-fill cements contain saddle dolomite, Fe-calcite, barite, sphalerite, quartz, and relict hydrocarbons. SEM, electron microprobe, and cathodoluminescence data suggest a highly complex fluid history within vugs and pores, with at least five generations of dolomite, calcite and silica cements. These cements are interpreted to range from early replacement of limestone to cross-cutting late diagenetic fracture- and vug-fill. The δ¹³C and δ¹⁸O stable isotope values from several of these late cements are isotopically light, suggesting interaction with hydrocarbon-bearing fluids during emplacement. The ⁸⁷Sr/ ⁸⁶Sr isotopic data suggest that the fluids which modified the crystalline dolomites beneath vuggy breccia zones are radiogenically enhanced, and therefore may have been externally derived. Paleomagnetic analyses of the Ellenburger indicate several chemical remanent magnetizations (CRMs). A karst breccia (conglomerate) test indicates that a late Ordovician CRM is secondary. It may be related to an early dolomitizing event. A Late Permian to early Triassic-age remagnetization is prevalent in several zones within the altered Ellenburger below the vug-bearing breccias. This CRM is interpreted to be related to basinally-derived fluids moving through paleoaquifers developed in open or undercemented portions of the karst units within the uppermost Ellenburger, about the time of maximum burial of the unit.