Fenestral and Associated Vadose Diagenetic Fabrics of Tidal Flat Carbonates, Middle Ordovician New Market Limestone, Southwestern Virginia

Abstract

ABSTRACT Tidal-flat facies of the Middle Ordovician (Chazyan) New Market Limestone (0-67 m), southwestern Virginia. are fenestral. pellet-intraclast packstone/wackestone and lime mudstone. The unit unconformably overlies Lower Ordovician dolomites (Knox Group), interfingers with laterally equivalent skeletal limestone and is overlain by skeletal wackestone (Lincolnshire Limestone). Fenestral fabrics consist of: (1) tubular fenestrae (0.1-2 mm dia.), subvertical, cylindrical and straight to curved; (2) laminoid fenestrae (0.1-1 mm high), parallel to curved and less than 15 mm long; (3) irregular fenestrae (0.5-5 mm dia.), equidimensional to irregular. Counts of the fenestrae in two measured sections indicates that all three fenestral types occur together forming complex fabrics; the fabrics are not cyclically repeated but are influenced by depositional fabric. Tubular fenestrae probably formed by burrowing organisms in soft, intertidal and/or shallow subtidal sediments. Irregular and laminoid fenestrae formed by desiccation/lithification in more frequently exposed sediments resulting in hard, indurated surfaces in tidal flats, which inhibited further burrowin . New Market fenestral fabrics suggest that tidal flat sedimentation kept pace with subsidence or sea level rise so that the depositional surface remained in tidal/supratidal zones for long periods, causing complex overprinting of fabrics. General absence of cryptalgalaminates reflects intense browsing/burrowing probably related to non-hypersaline tidal waters. Associated diagenetic features (abundant vadose silt; commonly pendant equant calcite cements, equant calcite cements which predate vadose silt; molds of shells; and solution-enlarged molds of shells) indicate early lithification and widespread vadose-meteoric diagenesis. Rare beds containing columnar (fibrous) calcite cements suggest precipitation of cements from more saline waters. Intra-and interformational erosional surfaces that truncate grains and diagenetic fabrics are further evidence of early lithification. Fenestral and associated vadose diagenetic fabrics, and the lack of dolomites, evaporites, intraclast breccias and cryptalgal sediments indicate that New Market sediments formed under the influence of non-hypersaline (normal marine to meteoric) tidal and ground waters and wet climatic conditions. Such fabrics may characterize other carbonate tidal deposits formed under similar conditions.