Biogenic Structures and Depositional Environments of a Lower Pennsylvanian Coal-Bearing Sequence, Northern Cumberland Plateau, Tennessee, U.S.A.

Sedimentary rocks of the Fentress Formation and lower Rockcastle Conglomerate include sandstones, shales, siltstones, and coals which are divisible into 6 facies. Facies characteristics, including rock type and physical and biogenic sedimentary structures, as well as vertical and lateral relationships between the facies, are consistent with interpretation of the sequence as deposited in back-barrier lagoons and tidal channels; tidal flats; and tidal inlet, channel, and delta environments. Superposition of burrowed sandstones, whose modem analogues are formed primarily in estuarine or marine environments, over coals in the middle of the sequence, and the occurrence of thinly bedded sandstones of tidal flat origin near the top, reflect the importance of marine influence during deposition of the sequence. Of the 20 ichnogenera found, 15 occur elsewhere only in rocks deposited in marine environments. Presence of characteristically marine trace fossils and absence of exclusively non-marine trace fossils give strong evidence of marine domination. This is corroborated by the lack of features characteristic of alluvial (channel or overbank) deposits or of sediments deposited on delta plains in strongly constructional, fluvially dominated delta systems. The Fentress Formation-Rockcastle Conglomerate sequence could have been deposited within a wave- or tide-dominated deltaic system or within a destructional portion of a broadly constructional delta, although an overall deltaic setting is not recognizable at the scale of this study.

Many trace fossils found in the sequence are restricted to rocks deposited in particular environments. Three assemblages are recognized: (1) a Palaeobullia-?Thalassinoides assemblage, occurring in tidal flat sandstones, (2) a Conostichus-Rosselia assemblage restricted to highly burrowed back-barrier sandstones, and (3) a ?Lennea assemblage including forms most abundant in sandstones interpreted as deposited on a lower tidal flat, perhaps adjacent or transitional to a tidal delta or channel.

Narrow facies ranges and marine affinities of trace fossils in these otherwise unfossiliferous rocks give further evidence that trace fossils are important aids in paleoenvironmental reconstruction.