The role of marine cementation in the preservation of Lower Palaeozoic assemblages

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.

Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas

“…Even in the absence of ready textural and faunal evidence of sea-¯oor lithi®cation at the ®eld level, there is evidence that primary porosity could be rapidly occluded on the Ordovician sea-¯oor by LMC cement growth. Ordovician and other Palaeozoic limestones from the eastern USA show a variety of criteria, including cement morphology, inclusion characteristics, cement/cement intergrowths, and cement/grain relationships that can be used to distinguish calcitic cements of sea-¯oor origin from later diagenetic (meteoric) ones (Walker & Diehl 1985;Walker 1989;Steinhauff 1989;Johnson & Goldstein 1999). Detailed studies in the Middle Ordovician of East Tennessee have shown that, at some levels, volumes of marine cements far exceed those of later diagenetic origin, and regularly account for 10% or more of total rock volume throughout sequences many tens of meters thick .…”

Relative rates of shell dissolution and net sediment accumulation ‐ a commentary: can shell beds form by the gradual accumulation of biogenic debris on the sea floor?

“…1986b, c;Westrop 1986;Peterson 1977). A high dissolution rate may not be the forcing term during some intervals of the Phanerozoic due to fluctuations in marine carbonate chemistry (Walker & Diehl 1985) and the preservation potential of 'event' beds (Brandt 1986). Nonetheless, during much of the geologic record, shell beds could not have commonly formed in most areas by the accumulation of shells at the sediment surface.…”