DOLOMITE TEXTURAL EVOLUTION AND FLUID DYNAMICS INVOLVING INTERCONNECTED AND HIGH PERMEABILITY FENESTRAL FABRIC IN EVAPORATIVE LAGOONAL SETTING OF FORT TERRETT FORMATION, EDWARDS GROUP, EARLY CRETACEOUS, CENTRAL TEXAS, USA

An outcrop of Fort Terrett Formation in Mason Area, Central Texas as South East end of Edwards Plateau represents a succession of dolomitized evaporative lagoonal facies interrupted by silicification of chert nodules. The formation overlies conformably the Hensel Formation (Hensel Sand) that those two sit on basement Town Mountain Granite unconformably.

The dominant very fine to fine dolomite with insertion of partially dolomitized mudstone-wackestone to packstone-grainstone and Rudist floatstone vertical succession characterizes subtidal to intertidal depositional environment, with fenestral porosity, ostracods and rootlets have been observed indicating occasionally supratidal setting. Three members consist of 8 units of the formation are identified, bounded by important stratigraphic surfaces.

Diagenetic history was affected by multiple episodes including marine, early meteoric, shallow burial and late meteoric/telogenetic diagenesis affected by high-frequency sea-level fluctuations, burial process and late stage exposure. The processes are supported by geochemical evidences such as increasing value of iron and manganese to lower member indicates increasing redox state in shallow burial environment to it and separation of stable isotope δ¹⁸O and δ¹³C distribution that points to different diagenetic processes.

Previous dolomitization model was proposed as seepage-reflux model by the texture of dolomite that requires ‘classic’ high permeability layers to have Magnesium-rich brines to flow, disperse or even diffuse outward of the formation and even downward to Hensel Sand through the muddy facies to have calcite replaced to dolomite. Interconnected fenestral porosities with high permeability properties observed throughout the succession are believed to be the main conduit for that dolomitization fluid in addition to that ‘classic’ layers.

Dolomite classification and grouping carry some genetic implication to textural evolution of dolomite. Four type of dolomites were observed and exhibit textural maturation toward crystal size coarsening and dolomite replacement and cementation related to decreasing of dolomitization fluid and/or dissolution of calcium carbonate and the fluid dynamic.