THE DEVELOPMENT OF A UNIQUE METHODOLOGY FOR POINT COUNTING AT OUTCROP SCALE AND ITS APPLICATION TO PALEOENVIRONMENTAL TRENDS IN A STROMATOPOROID-DOMINATED BIOSTROME IN THE SILURIAN KEYSER FORMATION, VIRGINIA

Point counting of a Silurian stromatoporoid-dominated reefal horizon at outcrop scale indicates that measurable paleoecological changes occur vertically in this biostrome, located in a cave in the Water Sinks, a karst feature in southern Highland County. Ranging in thickness from 1.5 to 2 m, it is the lower of two biostromes in the upper Keyser Formation, and is composed primarily of rudstones and framestones of stromatoporoids (S) and corals (notably Favosites (F), with lesser Cladopora (C)), and matrix (M). It overlies cross-bedded quartzose peloidal grainstones that contain variable crinoidal debris, and underlies massively bedded to cross-bedded crinoidal grainstones that also comprise much of the matrix of the underlying framestones. These strata have been partially dolomitized, with late stage ferroan baroque dolomite reducing primary interparticle porosity throughout.

Several standardized methods were synthesized into a unique methodology for point counting and data analysis at the outcrop scale, including a scaled-up version of standard methods as applied to thin sections. Data collected from each horizon were kept separate to allow detailed analysis of vertical succession within the biostrome. A 1.0 m wide x 1.5 m high section of the biostrome was point counted for framework grains and matrix at 5 cm intervals vertically and horizontally, for a total of 600 points. Length, width, morphology, and position of reef-building organisms were also recorded. Normalizing these point count data shows vertical variation in dimensions and abundances of reef builders. The average dimension of framework grains is 5.7 x 9.7 cm, although horizons at 0-25 cm and 50-100 cm have notably higher average dimensions of 9.5 x 12.5 cm and 7.2 x 12.9 cm, respectively. Abundances change from S₈₈F+C₅M₇ in the lower 50 cm; to S₆₆F+C₁₆M₁₈ in the middle 50 cm; to S₂₄F+C₃₄M₄₂ in the upper 50 cm, and stromatoporoids with low to high domical growth forms are predominant. Most are not in growth position, yet they do not appear to have undergone appreciable transport. Comparison of the Water Sinks biostrome with the well-known reef at Mustoe (12 km NNW and probably correlative with the upper rather than the lower biostrome) suggests that the Water Sinks biostrome has lower diversity, and is a reef flank or small reefal buildup rather than a true reef.