No abstract

Many workers have emphasized recently the taphonomic processes involved in the preservation of fossil assemblages, and some have doubted that invertebrate assemblages have ever been preserved that reflect closely the living community. Yet there are many examples in the literature in which invertebrate assemblages have been interpreted in just such a fashion. Many of the most confident reconstructions have been based upon assemblages in early Palaeozoic limestones. What was it about some early Palaeozoic environments that led at times to such exquisite preservation? Carbonate petrologists have stressed the importance of marine cements in some modern environments (especially reefs). Our observations in Ordovician shelf carbonates suggest that such marine cementation may have been more common in a wider range of environments in the early Palaeozoic. The reason was a sufficiently different marine water chemistry at that time. Marine cementation effects accumulating biotic assemblages in three ways: (i) the formation of true hardgrounds with establishment of a specialized biota on the completely lithified surface; (ii) production of peculiar ‘mixed’ assemblages by incremental, patchy hardground formation during accumulation leading to a mixture of encrusting forms in the same bed as loose-sediment forms; and (iii) preservation of fossils in life position, or retardation of transportation and mixing, caused by early immobilization of skeletons by cement fringes. Our thesis is that early Palaeozoic carbonate shelf communities are better preserved than those of other environments and other times because the fossils were more often ‘locked’ in place by very early, marine cements.

The “Thorn Hill Section” is an amazingly complete and well exposed section of rocks that has served at least four generations of geologists as a reference for early Cambrian through early Carboniferous stratigraphy of the Southern Appalachians. Eastward from War Ridge along 8.8 mi (14 km) of U.S.25-E, across Clinch Mountain to Poor Valley Ridge, Grainger County, Tennessee, virtually every unit between the Rome and the Grainger formations in the Southern Appalachians is exposed in the 10,912 ft (3,326 m) thick sectio (Fig. 1). The section is bounded by two major thrust faults-the Copper Creekfault and the Saltville fault; the Rome Formation is the hanging wall forboth faults. Most of the section is embraced by the Avondale, 7½-minute topographic quadrangle with the remainder on the Dutch Valley, 7½-minute Quadrangle. The stratigraphic descriptions given here are synopses from Walker (1985). The stratigraphy here represents a westward thinning edge of a thick wedge dof Paleozoic sedimentary rocks whose history is recorded in three stratigraphic sequences bounded by unconformities of regional extent (Fig. 2). Sequence I embraces Cambrian through Early Ordovician, Sequence II the middle Ordovician through Early Devonian, and Sequence III Late Devonian through Early Mississippian. The stratigraphic descriptions given here are synopses from Walker (1985). The stratigraphy here represents a westward thinning edge of a thick wedge of Paleozoic sedimentary rocks whose history is recorded in three stratigraphic sequences bounded by unconformities of regional extent (Fig. 2). Sequence I embraces Cambrian through Early Ordovician, Sequence II the middle Ordovician through Early