Hardgrounds

Hardgrounds are "synsedimentarily lithified carbonate seafloor that became hardened in situ by the precipitation of a carbonate cement in the primary pore spaces" (Wilson and Palmer 1992:3). Thus, hardgrounds are not necessarily associated with very high hydrodynamic energy.

Hardgrounds occurred for the first time in geologic history not earlier than late Middle Cambrian. Since the Ordovician, hardgrounds have occupied locally extensive areas on the sea floor and have been characterized by an abundant and diverse benthic fauna. Hardgrounds may pass laterally to various debris-rich soft grounds, resulting in the existence of mixed hardground and softground associations.

Wide distribution of hardgrounds from the beginning of the Ordovician can be largely explained by abiotic factors (Wilson et al. 1992; Myrow 1995), the most important of which was lowering of the Mg2+/Ca2+ ratio and rise of CO2 activity in sea-water, which can account for change in mineralogy of marine carbonate precipitates. This resulted in the replacement of shallow-water high-magnesium calcite and arag-onite precipitation by low-magnesium calcite: so-called aragonite seas were replaced by calcite seas (Sandberg 1983). The original calcite cement grew syntaxially on calcite substrates such as echinoderm ossicles and other calcite bioclasts; early aragonite cement could not do this. Although hardgrounds occur in aragonite seas as well (e.g., at the present day), they appear to have been more widespread in calcite sea times because calcite precipitates faster and more extensively (Wilson and Palmer 1992).

The structure of the echinoderm skeleton is another factor promoting hardground formation, through a significant increase in calcite debris on the sea floor (Wilson and Palmer 1992). First, it is highly porous and hence achieves considerably greater volume for the same weight in comparison with calcite skeletons of other animals. Second, the skeletons of echinoderms are built from separate small skeletal elements joined together by organic ligament. This construction allows rapid postmortem disarticulation and fragmentation of the skeleton, with the accumulation of large amounts of debris on the sea floor. For example, after death and fragmentation, a crinoid skeleton with height of 1m and a stem diameter of 0.5 cm could produce enough debris to cover at least 0.5 m2 of sea floor with a layer 1 mm thick.

A certain balance between sediment deposition and lithification is necessary for hardground formation. When sedimentation was faster than lithification, a particular kind of softground with a hardened underlying layer was formed. This phenomenon is responsible for a wide variety of semihard substrates and for their various combinations with true hardgrounds, which has resulted in a high diversity of benthic fauna inhabiting these substrates, as can be observed, for example, in the Early Ordovician of the Baltic paleobasin (Rozhnov 1994).

CHARACTERISTICS OF THE EARLY PALEOZOIC SEA FLOOR Marine Substrates in the Cambrian

The Cambrian sea floor was covered mainly with soft silt sediments, whereas deposits enriched with bioclastic debris were rare (see also Droser and Li, this volume). In the Early Cambrian, firm bottoms occupied small areas and were represented almost entirely by rockgrounds. Rockground faunas are poorly known on account of their poor preservation.

Nevertheless an unusual fauna was discovered in calcimicrobial-archaeocyath reefs of western Nevada and Labrador (James et al. 1977; Kobluk and James 1979): calcified cyanobacteria, sponges (including juvenile archaeocyaths), possible foramini-fers, some problematic organisms, and Trypanites borings. These organisms inhabited reefal cavities that were completely or partially protected from wave action. A similar cryptic fauna has been found in cavities of Early Cambrian reefs in many regions of the world, including the Siberian Platform, southern Urals, Altay Sayan Foldbelt, Mongolia, southern Australia, and Antarctica (Zhuravlev and Wood 1995).

Hardgrounds formed by early diagenetic replacement of cyanobacterial mats by phosphatic minerals are known from the Middle Cambrian of Greenland. Numerous small echinoderm(?) holdfasts are attached to these hardgrounds (Wilson and Palmer 1992).

The earliest typical hardground surfaces, with numerous eocrinoid holdfasts and some orthid brachiopods and spicular demosponges, have been found in the late Middle Cambrian part of the Mila Formation in the Elburz Mountains, northern Iran (Zhuravlev et al. 1996) (figure 11.2). In this example, hardgrounds developed on cal-ciate brachiopod shell beds and lithified bacterial (algal?) crusts. Eocrinoid settlement on carbonate flat pebbles is described from intraformational conglomerates of the Late Cambrian of Nevada, Montana, and Wyoming (Brett et al. 1983; Wilson et al. 1989). Such rigid bottoms can be considered as genuine hardgrounds, though they differed in some aspects from Ordovician hardgrounds (Rozhnov 1994).

Thus, in the Cambrian there were no close similarities between the faunas of rock-grounds and the first hardgrounds. However, Trypanites may provide an exception, because the most ancient borings of these animals are found in Early Cambrian reefal

Figure 11.2 Hardground surface with eocrinoid holdfasts, collection of PIN, late Middle Cambrian Mila Formation, Member 3 (Shahmirzad, Elburz Mountains, northern Iran). Source: Photograph courtesy of Andrey Zhuravlev. Scale bar equals 1 cm.

cavities of Labrador and western Newfoundland (James et al. 1977; Palmer 1982). These borings are not known from the Middle and Late Cambrian (Wilson and Palmer 1992) but reappear in great numbers in Early Ordovician hardgrounds (Rozhnov 1994), becoming widespread in the Middle and Late Ordovician. However, the real identity of the progenitors of Early Cambrian and Ordovician Trypanites raises some doubts, because the Ordovician borings are considered to have been produced by polychaetes, whereas the nature of Cambrian Trypanites remains unknown ( James et al. 1977; Kobluk et al. 1978). Thus, one can suppose that the majority of the hard-ground fauna arose independently of the rocky bottom fauna. Attached echino-derms are pioneers and are the most important components of the initial hardground ecosystems.

Cambrian hardgrounds were created presumably by consolidation of cobbles or large shells, on which echinoderms initially settled (Brett et al. 1983; Zhuravlev et al. 1996). The debris, accumulated between pebbles after postmortem destruction of echinoderm skeletons, favored cementation of pebble bottoms. Calcite productivity of echinoderms in the Cambrian was low, and the debris produced by echinoderms was only enough to fill spaces between cobbles. Thus, the community that settled on such hardgrounds could not expand the hardground area beyond the pebbled area. The low abundance of hardgrounds in the Cambrian was determined by these limits and also probably by the reduced distribution of calcite seas at that time.

Marine Substrates in the Ordovician

A considerable part of the Ordovician epicontinental sea floor was also covered with soft silts. Ordovician soft substrates, however, in contrast to the Cambrian ones, commonly contained abundant calcite debris and thus were transformed into hard-grounds that occupied large areas.

Ordovician as well as Cambrian rockgrounds occupied relatively small areas and were colonized only by benthic animals to a limited extent. Abundant and diverse faunas largely developed in framework cavities within various reefs. The framework cavities in bryozoan-algal reefs (Middle Ordovician, Caradoc) from near Vasalemma village in Estonia provide an example; various bryozoans, crinoids, cystoids, edrioa-steroids, and brachiopods, often well preserved, are found in these cavities (pers. obs.). Nonetheless, on the whole, this fauna was insignificant for the evolution of the marine benthos, because such ecologic niches were relatively ephemeral, their colonization was rather occasional, and they had no evolutionary future.

Ordovician hardgrounds were very widely distributed. They occupied large areas and were colonized by a characteristic and abundant fauna. This was especially typical of Middle Ordovician hardgrounds (Palmer and Palmer 1977). The faunas of Early Ordovician hardgrounds are considered to be transitional between those of Cambrian and Middle Ordovician hardgrounds, based on detailed analysis of hardgrounds in the Middle Ordovician Kanosh Shale in west-central Utah (Wilson et al. 1992). The formation of hardgrounds in the carbonate part of this sequence can be described by the following succession of steps (Wilson et al. 1992): (1) development of early dia-genetic carbonate nodules in fine-grained siliciclastics; (2) storm current winnowing and formation of cobble lags; (3) encrustation of the cobbles by large numbers of stemmed echinoderms (predominantly eocrinoids), trepostome bryozoans, and a few sponges; (4) accumulation of echinoderm debris in lag deposits; (5) and early marine cementation of hardgrounds and the settlement of additional stemmed echinoderms, bryozoans, and sponges.

The community of the third stage of this sequence can be compared with the Late Cambrian community (Rozhnov 1994) found in the Snowy Range Formation of Montana and Wyoming (Brett et al. 1983), as well as with the late Middle Cambrian community of the Mila Formation of Iran. All these communities are similar in the dominance of eocrinoids and the absence of Trypanites borings, which are typical of younger hardgrounds.

The presence of bryozoans in Early and Middle Ordovician hardground communities is considered the main ecologic difference from Cambrian hardground communities. In my opinion, however, the most important difference between these hard-grounds is displayed in the mechanism of their formation. Cambrian hardgrounds developed only on pebbles (Snow Range Formation) or large calciate brachiopod shells (Mila Formation), because calcitic debris from echinoderms and other en-crusters was sufficient only to fill the space between the pebbles, whereas in the Early

ment. Source: Modified after Rozhnov 1994.

Ordovician, as demonstrated for the Kanosh Shale, the amount of echinoderm debris was enough for hardground formation even outside the area covered by pebbles (Wilson et al. 1992). Therefore, the analogs of the fourth and fifth stages of development of hardgrounds in the Cambrian described by Wilson et al. (1992) and Zhuravlev et al. (1996) were absent, and these stages can be considered as typically Ordovician phenomena (figure 11.3). The accumulation of abundant debris, initially provided by echinoderms, and fast expansion of these hardgrounds due to the supply of debris coming from new encrusters, are characteristic of these later stages (figure 11.3).

Study of Early Ordovician hardgrounds from the eastern part of the Leningrad Region (Baltic Basin) has revealed further differences from Cambrian hardgrounds and provides an opportunity to establish a pattern of hardground formation based on positive feedback between the development of encrusters (initially echinoderms), and the expansion of hardgrounds themselves (Rozhnov 1994, 1995; Palmer and Rozh-

Figure 11.4 Positive feedback between the expansion of hardgrounds and the increase of calcite debris production by hardground communities.

nov 1995) (figure 11.4). One of these features is the presence of Trypanites borings, widely distributed in Early Ordovician hardgrounds of the Baltic Basin, as has already been reported by Hecker (1960) and Vishnyakov and Hecker (1937). The second important difference is the mass supply of debris, produced mainly by echinoderms inhabiting hardgrounds, and its accumulation in areas where new hardgrounds or soft grounds, depending on the sedimentary regime, possessing a hard layer at a given depth below soft sediments were formed.

Hardgrounds could not develop widely in the Ordovician until the quantity of accumulated calcite debris on the sea floor increased sharply in comparison with that of the Cambrian. This increase in debris supply in the Ordovician was, first of all, connected with the change in the structure of benthic communities, especially in the carbonate-precipitating seas, where echinoderms began to play a dominant, or at least an important, role. The abrupt increase in the amount of echinoderm debris in postCambrian sediments corroborates this opinion.

Supply of calcite debris produced by other groups of animals, such as ostracodes, brachiopods, bryozoans, and trilobites, also sharply increased in the Ordovician. This implies that the production and supply of CaCO3 debris by various organisms in the Ordovician increased. In any case, the balance of CaCO3 content in marine water should have been affected because of the redistribution of its production among different groups of organisms (from mostly trilobites in the Cambrian to echinoderms, brachiopods, bryozoans, and mollusks in the Ordovician) (see also Droser and Li, this volume).

In the Ordovician, echinoderm calcite productivity increased by at least an order of magnitude relative to that in the Cambrian. It was connected with an increase in the general number and variety of echinoderms, as well as with their individual increase in size. In the Cambrian, stemmed echinoderms were represented mainly by eocrinoids, which almost never reached a height greater than 15 cm above the sea floor and usually were shorter (Bottjer and Ausich 1986; Ausich and Bottjer 1982; Rozh-nov 1993).

In the Ordovician, some eocrinoids reached a height of 25-30 cm (Rozhnov 1989), and crinoids with long stems could rise 1 m or more above the sea floor. The diverse

Figure 11.5 Maximum height of food-gathering apparatus above the sea floor among some groups of benthic animals in the Cambrian and Ordovician. Source: Modified after Rozhnov 1993.

and numerous cystoids could reach 30-40 cm in height (figure 11.5). This resulted in the deployment of suspension feeding into the basal meter of the water column. It sharply increased the tiering for echinoderms and, as a consequence, caused an increase in the overall number of echinoderms. Simultaneously, the individual sizes of echinoderms sharply increased by almost an order of magnitude. This was connected not only with the replacement of small-sized groups by larger ones but also with a general trend of size increase in all groups of echinoderms. Large crinoids were common in the Ordovician and often formed dense settlements. As a result of these developments, supply of calcite debris to the sea floor increased dramatically.

Therefore, substrates around such settlements mostly consisted of echinoderm debris. Not far from these settlements, echinoderm debris also constituted a substantial proportion of the sediment. For example, as described by Polma (1982) in the Ordovician of the northern structural-facies province of eastern Baltica, echinoderm fragments compose 25-30 percent of the total amount of debris, increasing in reefal facies up to 95 percent. Such a change in the character of substrates at the Cambrian-Ordovician boundary would likely affect the structure and diversity of the entire benthos. Another feature of echinoderms that influenced sea floor changes in carbonate-precipitating seas at this boundary that should be taken into account is that each skeletal element of an echinoderm is monocrystalline. Calcite cements grew syntaxi-ally on isolated echinoderm ossicles, and thus the cementation rate in sediments enriched by echinoderm debris was very fast. As a result, in suitable conditions abundant echinoderm debris was rapidly cemented on the sea floor to form hardgrounds

(Wilson et al. 1992). When the rate of sedimentation was equal to, or less than, the rate of cementation, substrates became rigid and hardgrounds formed. These new hardgrounds were ideal for the settlement of stemmed echinoderms that needed rigid substrates, and they quickly colonized them. Hardgrounds were also favorable for the settlement of many other benthic groups, such as bryozoans, ostracods, and small brachiopods, as well as for boring organisms, among which Trypanites dominated.

Was this article helpful?

0 0

Post a comment