Abstract

The Finis Shale Member (Upper Pennsylvanian, Virgilian, Cisco Group, Graham Formation; Barnes 1987) in the Jacksboro-Graham area is situated in the lower middle part of a well preserved transgressive-regressive cycle, the Finis Shale Cycle. Below it the cycle begins with a lenticular sandstone, a mudcracked greenish shale and a poorly developed transgressive limestone correlative with the Salem School Limestone Member of the Graham Formation. Exceptionally well preserved in situ benthic invertebrate fossil assemblages in this cycle allow for a detailed paleoecological investigation, providing information about possible past environmental conditions relating to the water column and substrate. Marine benthic communities preserved within the Finis Shale Cycle begin with a very shallow water Myalina community, a very thin (10 cm) mudstone containing a Triticites community followed by a deeper water, relict mature Crurithyris-Paraconularia community, a Crurithyris-Glabrocingulum community and a relatively mature Hustedia-Phymatopleura community which persists through the transgressive-regressive shift (and disappears a few decimetres above). A Rhipidomella-Dentalium community is topped by a thick (30 cm) interval of abundant Triticites within the regressive portion of the cycle. The base of the Jacksboro Limestone tops the section. The described community changes could be interpreted as primarily due to changes in sea level, sediment supply and energy input. However, only 20 miles to the southwest fissile, poorly fossiliferous black shales dominate the lower part of the Finis Shale, suggesting that lateral pycnocline shifts may also have influenced living conditions of benthic communities.