Mechanism Of Hardground Feedback

Ordovician hardgrounds were formed by the accumulation of calcite debris produced by a benthic community inhabiting the very same substrate (Wilson and Palmer 1992). In the Cambrian there was a similar source of debris supply, but the quantity of debris was not sufficient for the expansion of hardgrounds. That is, hardgrounds could appear under suitable conditions, usually when echinoderms settled on cobble lag surfaces, but they could not expand beyond these lags. In contrast, Cretaceous hardgrounds depended on debris of planktic organisms, mainly coccolithophorids. Thus, the phenomenon of early Paleozoic hardground feedback lies in the ability of hardground expansion, which depends on the amount of debris supplied by the ben-thic community itself. This phenomenon is especially typical of the Ordovician.

Hardgrounds with Low and Medium Hydrodynamic Energy

The feedback mechanism of Ordovician hardgrounds is connected primarily with echinoderms, for which hardgrounds with low and medium hydrodynamic energies represented ideal locations for settlement. The echinoderm larvae were planktic and became attached to some hard surface for further development—for example, to

Figure 11.6 Hardground with encrusting crinoid holdfast, bryozoans and Trypanites borings, PIN 4565/10, Early Ordovician, Arenig, Volkhov stage (Simonkovo village, Volkhov River right bank, Leningrad region). Natural size.

large bioclasts on softgrounds or to hardgrounds (Guensburg and Sprinkle, this volume). In addition, long-stemmed echinoderms with high crowns, such as crinoids, needed a sufficiently strong support on the sea floor. Stemmed echinoderms easily solved this problem by attaching to hardground surfaces by the simplest, primitive holdfast (figure 11.6). On softer substrates this primitive holdfast was considerably complicated by a ramose root system (figure 11.7).

Echinoderm Debris as a Material for Hardgrounds

Echinoderm debris accumulated around echinoderm communities and was an ideal material for the formation of hardgrounds because of the following features: (1) the single-crystal nature of each echinoderm skeletal element, resulting in fast syntaxial growth of calcite cement precipitating from pore waters in sediment on loose echino-derm debris; (2) the larger volume of echinoderm debris in comparison with calcite debris of the same weight produced by other animals, due to the high porosity of the echinoderm skeleton (stereome structure); and (3) the multiple nature of the echino-derm skeleton, resulting in the postmortem production of numerous calcite ossicles

Figure 11.7 Ramose crinoid holdfast on the surface of a softground (grainstone), PIN 4565/11, Early Ordovician, Arenig, Volkhov stage (Obukhovo village, Volkhov River right bank, Leningrad region). Natural size.

even in quite quiet water. Thus, hardgrounds represented an ideal place for dense settlement of echinoderms, and the postmortem accumulation of their debris favored further hardground development (Guensburg and Sprinkle 1992; Wilson et al. 1992). The more hardgrounds expanded, the greater was the number of echinoderms that settled on their surfaces, and the more calcite debris accumulated around these settlements, which further enhanced hardground expansion. This positive feedback between the expansion of hardground areas and the increase of echinoderm biomass led to very rapid expansion of hardgrounds over large areas and to abrupt and rapid increase in the number of echinoderms and of other sessile organisms (see figure 11.4) because it occurred in shallow-water calcite-precipitating Ordovician seas.

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