Rank exposure index on a Silurian carbonate tidal flat



Adequate discrimination between ancient intertidal and supratidal limestones and dolomites is often difficult because of vagueness of terms, problems of resolution, and the fact that most ‘diagnostic’sedimentary features are not restricted to narrow environmental zones. Furthermore, the rise and fall of sea-level in restricted, epeiric seas is usually due more to storm surges than to true tides, and hence the occurrence and extent of tidal-flat inundations are quite irregular. ‘Rank exposure index’is a concept introduced (1) to quantify the relative percentage of time that sediments are subaerially exposed and (2) to produce a more refined subdivision of shoreline carbonate rocks. The scheme employed to determine this index is a combination of numerical methods: cluster analysis separates 46 Tonoloway Limestone samples (Silurian of Maryland) into groups based on 16 sedimentary, mineralogic, and organic features, and minimum spanning tree produces a connected series of the five resultant groups. These groups are then ranked in sequence from 1 (greatest subaerial exposure) to 5 (least). Rocks of rank 1 are thought to have been deposited at the highest position within the tidal zone, as on a beach ridge. Rocks of decreasing rank reflect greater frequency of flooding. Those of rank 5 represent sediments deposited closest to low-water level, as in a tidal pond. Rock type, mineralogy, fossil content, algal stromatolites, and sedimentary structures are closely related to exposure rank as interpreted for the Tonoloway carbonates. Rank exposure index, therefore, is a semi-quantitative measure to aid in the detailed reconstruction of ancient shoreline environments.