Abstract: Fenestral Fabrics in Noncyclic "Low" Salinity Tidal-Flat Carbonate Rocks, Middle Ordovician, Virginia

George Grover, Jr.

Tidal-flat facies of the Middle Ordovician New Market Limestone, Virginia, contain complex fenestral fabrics and distinctive vadose diagenetic fabrics. New Market fabrics may characterize an important group of early Paleozoic tidal-flat facies that formed under the influence of normal marine to brackish tidal/ground waters.

The New Market Limestone (0 to 200 ft or 61 m) unconformably overlies Lower Ordovician Knox Group dolomites and consists mainly of fenestral pellet/intraclast packstone, and lime mudstone; it is overlain by Lincolnshire Limestone skeletal wackestone. Fenestral fabrics are dominated by three distinct but complexly superimposed fenestral types. Tubular fenestrae (0.1 to 0.5 mm wide) are subvertical, cylindrical, and commonly bifurcate; laminoid fenestrae (0.1 to 1 mm high) are parallel to curved and less than 15 mm in length; irregular fenestrae (0.5 to 5 mm dia.) are equidimensional to irregular. Cryptalgalaminates are rare. Point-count analyses of vertical sequences of fenestral fabrics indicate that fabrics are not repeated cyclically.

Origin of fenestral types is difficult to determine where they are superimposed completely. However, in correlative cyclic tidal-flat facies in westerly belts, tubular fenestral fabrics overlie subtidal skeletal units and underlie mud-cracked cryptalgalaminates; this suggests that tubular fenestrae are burrows formed in little-exposed intertidal environments; cryptalgalaminates probably developed in frequently exposed tidal environments. Absence of cryptalgalaminates in much of the New Market Limestone may reflect intense browsing/burrowing related to nonhypersaline tidal waters. Laminoid and irregular fenestrae probably formed by alternate wetting/drying and incipient lithification due to frequent exposure. Complex superimposed fenestral fabrics suggest superimposed diagenetic enviro ments as tidal sedimentation kept pace with subsidence; in contrast, cyclic units in westerly belts suggest tidal-flat progradation alternating with marine submergence. Nonhypersaline tidal/ground waters during New Market deposition are indicated by abundant vadose silt, leached fossils, early radial-granular to blocky cements predating vadose silt, and by the lack of evaporites,cryptalgal structures, intraclast breccias and dolomite. Fenestral and associated diagenetic fabrics of the New Market Limestone may be developed in many other early Paleozoic tidal-flat carbonate rocks that formed under nonelevated salinities.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA