Ecostratigraphy of the Lower Middle Ordovician and paleoecology of Porterfieldian and Wildernessian biotas at Evans Ferry, Tennessee

This study is an ecostratigraphic and paleoecologic analysis of a part of the complex facies in Middle Ordovician carbonate shelf sediments at Evans Ferry, Tennessee. The 330 meter stratigraphic section begins at the Post-Knox unconformity and comprises in ascending order: the Tumbez Formation, the Eidson and Hogskin members of the Lincolnshire Formation and the Rockdell, Benbolt and Wardell formations. This study consists of two parts. . First, a petrographic analysis of 146 thin sections from the stratigraphic section to establish a paleoenvironmental framework. Using this framework, two fossiliferous intervals (A and B), representing different environments, were selected for paleoecologic study.

Deposition of the Evans Ferry section began in the supratidalintertidal environment, progressed to a stable offshore subtidal environment and ended with a return to peritidal conditions. The Tumbez Formation represents a supratidal-high intertidal environment at its base and a low intertidal algal flat environment at its top. Algal flats were maintained throughout the deposition of the lower Eidson Member, but were succeeded by shallow subtidal conditions in the middle and upper units. Stable subtidal conditions are inferred for the deposition of the Hogskin Member with occasional interruptions by storm or tidal channel deposits. . The Rockdell Formation was deposited in a subtidal environment with semirestricted water circulation. The upper units of the Rockdell Formation and the lower units of the Benbolt Formation represent interacting stable subtidal environments characterized by intermittent deposition of mud and decreasing water depth. The remainder of the Benbolt Formation includes deposits of tidal skeletal sand bars, algal mud mounds and deeper subtidal intermound areas. A gradual change from shallow nearshore subtidal to high intertidal-supratidal environments is inferred for the Wardell Formation.

Interval A composes the lower 2l meters of the Benbolt Formation (l7l.3-l92.3 meters from the base of the section) and is divisible into three lithologic zones based on the packstone to shale (pk:sh) ratio. Zone I (9 m thick) has a 2:l ratio and exhibits the highest species variety of any studied interval. Fossil assemblages represent gradual environmental changes, and are dominated by stenohaline epifaunal suspension feeders. A parameter was devised (uniformity index), which characterizes the distribution of species within fossil Classes. Zone I has high uniformity indices indicating a monotonic decrease in numbers of species within Classes.

Zone II (4.5 m thick) has a l:2 pk:sh ratio, lower species variety, and low uniformity indices suggesting less environmental stability. Abundance of epifaunal suspension feeders decreases concomitant with an increase in vagile deposit feeders and browsers. Zone III (7.5 m thick) has a variable pk:sh ratio of 5:l at the base to 2:l at the top. The less exposed and weathered outcrop of zone III may account for the few species identified other than in thin sections, and indicates

a large sample collecting bias. All of interval A represents a stable offshore, level bottom, subtidal environment.

Interval B contains 47.2 meters of rock of which the lower 26.4 meters is fossiliferous; the remainder is composed of intertidal mudstone and calcareous shale laminae. The fossiliferous rock sequence contains five paleocommunities, which represent an offshore to onshore regression, these are: (1) Hesperorthis community; (2) Strophomena- Hesperorthis community; (3) Tetradium-Girvanella community; (4) Chaulistomella-Stophomena community; and (5) Strophomena-Rostricellula community. The Strophomena-Hesperorthis assemblages resemble an ecotonal community between the Hesperorthis community (furthest offshore) and the Tetradium-Girvanella wave baffle community. The Chaulistomella- Strophomena and Strophomena-Rostricellula assemblages represent shallow nearshore subtidal communities interstratified with intertidally deposited sediments.

Total species diversity of the biota in interval B decreases up the section (toward nearer-shore environments). The uniformity indices for the communities are very low reflecting the dominance of species variety by one class. The biota displays two trends with decreasing distance from shore, which are: (l) a progressive change from sessile suspension feeders to vagile deposit feeders and (2) a change toward increasing salinity tolerances. Consequently, interval B represents communities exhibiting compositional changes in response to increasingly harsh physical conditions.