Point counting at the outcrop scale: Application to paleoenvironmental trends in stromatoporoid biostromes, Silurian Keyser Formation, Highland County, Virginia

Selina R. Cole¹, John T. Haynes¹, Philip C. Lucas², Richard A. Lambert³, and Stephen A. Leslie¹
¹James Madison University, Harrisonburg, VA; [email protected], [email protected], [email protected]
²Virginia Speleological Survey, Burnsville, VA, [email protected]
³Virginia Speleological Survey, Monterey, VA, [email protected]

Based on point count data collected from three exposures in southern Highland County, Virginia, quantifiable paleoecological changes that occur vertically are documented in two stromatoporoid-dominated reefal horizons in the Silurian upper Keyser Formation. The 1.5-2 m thick lower biostrome, exposed in the Water Sinks Subway Cave, is composed primarily of rudstones of stromatoporoids (S), 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 biostrome. Much of the Keyser at this exposure has been partially dolomitized, with ferroan baroque dolomite being a common late stage cement. In the Old Water Sinks Cave, the 4-5 m thick upper biostrome is comprised primarily of framestones of larger stromatoporoids and tabulate corals, many of which are in growth position. The upper biostrome has higher species diversity than the lower biostrome, with common rugose corals and bryozoans in addition to large stromatoporoids and tabulate corals. The stromatoporoid buildup at a surface outcrop near Mustoe, ~6 km NW of the Water Sinks, is correlative with the upper biostrome at the Water Sinks but is thicker and more biohermal in nature than either of the biostromes in the caves at the Water Sinks.

Standard point counting methods used for quanitative analysis of thin sections were scaled up for use at the outcrop level. A 1.0 m wide x 1.0 m high section of each biostrome was point counted at 5 cm intervals for framework grains and matrix, vertically and horizontally, for a total of 400 points. Length, width, morphology, and orientation were recorded for reef-building organisms. Average abundances of the lower biostrome in the Subway are 59% stromatoporoids, 23% matrix, 16% Favosites, and 2% Cladopora. Dominant stromatoporoid forms are low domical (29%), broken (26%), and high domical (26%), while other growth forms comprise less than 20%. Average dimensions for stromatoporoids are 5.8 cm high x 9.7 cm wide. Orientation of stromatoporoids is 99% rotated and 1% in place. Only 0.2% of stromatoporoids show an encrusting habit. The Mustoe reef core data show average abundances of 87% stromatoporoids, 7% matrix, 3% Favosites, and 3% Cladopora. Bulbous (24%), tabular (23%), and high domical (20%) forms dominate. Average dimensions for stromatoporoids in the core are 12.4 cm high by 16.9 cm wide. Orientation is 33.5% rotated and 66.5% in place, and 24% of the stromatoporoids show encrusting habits. Based on high matrix content and the abundance of rotated stromatoporoids, comparison of the Water Sinks Subway biostrome with the reef at Mustoe suggests that the lower biostrome is dominated by transported reefal specimens, classifying it as a reef flank or small reefal buildup rather than a true reef.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012