PALEOMAGNETIC TIMING OF DIAGENESIS IN THE UPPER ELLENBURGER GROUP CARBONATES SUGGEST A PERIODICALLY OPEN DIAGENETIC SYSTEM, FORT WORTH BASIN, TEXAS

A study of Ordovician Ellenburger Group carbonates in the subsurface of the Fort Worth Basin, Texas, provides a unique view of the evolution of a periodically open diagenetic system through time. Data suggest the rocks were diagenetically altered and remagnetized via fluid alteration. Thermal demagnetization of samples from oriented cores reveals a low-temperature steeply downward viscous remanent magnetization (VRM) as well as a characteristic remanent magnetization (ChRM) removed at higher temperatures (200-540°C) with specimen directions that are streaked from an easterly and shallow direction to a southerly and shallow direction. The ChRM resides in magnetite and is interpreted as one or more chemical remanent magnetizations (CRMs) based on low burial temperatures. The VRM was used to orient the CRM data for one of the wells and to test the scribe orienting method. The results confirm that the streak of directions is real. One interpretation is that the streak represents a mixing trend between two or more components from Ordovician to Triassic. Another possibility is that the streak was caused by a possible tectonic counterclockwise rotation of the basin by 15 to 20°. Grouping of the specimens by diagenetic facies reveals that the streak may be composed of distinct components. Specimens from Karst clasts fail a paleomagnetic ‘conglomerate test’ and contain an Ordovician chemical remanent magnetization (CRM) in magnetite that is interpreted to be related to fluid alteration. A shaly dolomite-limestone facies with vertical solution fractures at the top of the Ellenburger contains a paleopole which falls off of but close to the Late Mississippian-Early Pennsylvanian part of the apparent polar wander path. It is interpreted to represent alteration by fluids, perhaps related to the burial of the overlying Barnett shale. In some fractured dolomites, a Late Permian-Early Triassic CRM occurs in zones with elevated ⁸⁷Sr/⁸⁶Sr values, suggesting that remagnetization is related to fluids derived from the nearby Ouachita thrust. Dolomites with natural fractures, calcite-filled vugs, fluid inclusions with warm saline fluids, and radiogenic ⁸⁷Sr/⁸⁶Sr values contain a Late Triassic-Jurassic CRM that may be related to late gravity-driven fluid migration off the Ouachita uplands.