Reefbuilding Agents

Reefs consist basically of a rigid framework around which collect calcareous sediments generated by the reef itself. For this reason, reefs are not restricted to structures with conspicuous metazoan frame builders; stromatolites and thrombolites compose reefs too (Pratt and James 1982; Pratt 1982, 1995). Since all reefs possess topographic relief, we do not distinguish between reefs and mounds, unlike James and Bourque (1992). In the Cambrian, reefal framework was mostly the product of accreting mi-crobial elements encrusted by animal skeletons, but locally dense settlements of skeletal animals strengthened by microbial crusts and synsedimentary cementation. Reef matrix sediment contains fragments of this framework, bioclasts of animals that lived on and under it, along with peloids and lime mud. Early Cambrian reefs are better studied than reefs of many younger intervals, and detailed descriptions can be found in a number of papers (James and Kobluk 1978; Kobluk and James 1979; Rees et al. 1989; James and Gravestock 1990; Debrenne et al. 1991, 1993; Kruse 1991; Wood et al. 1993; Kruse et al. 1995, 1996; Zhuravlev and Wood 1995).

Stromatoid-Thromboid Associations

Among Cambrian reefs, laminated and unlaminated microbial structures—stromatolites and thrombolites, respectively—are common (Pratt 1995). They are composed of fenestrate micrite and clotted micrite believed to have originated through organically induced precipitation within microbial biofilms or mats. Like modern counterparts, these mats were dominated by photosynthetic taxa but may have contained heterotrophs as part of a complex microbial community of cyanobacteria and other bacteria. CaCO3 precipitation likely occurred during mat decay, and only rarely is some evidence of the mat-dwelling "cells" preserved (Pratt 1995: figure 6D).

Thromboids are generally subordinate elements in Early Cambrian reefs (figure 12.1A), but are usually dominant in those of Middle and Late Cambrian age from shelf

Figure 12.1 Thin-section photomicrographs of Lower Cambrian frameworks. A, Upside-down bowl-shaped Metaldetes profundus (Billings) encrusted on one side by thromboid (t) and on the other side first by Girvanella (g), second by cryptic Archaeotrypa sp. (c), and third by Renalcis; right side of Metaldetes perforated by Trypanites macroboring (b); stick-shaped Metaldetes at right and Retilamina amourensis Debrenne and James at lower left

Figure 12.1 Thin-section photomicrographs of Lower Cambrian frameworks. A, Upside-down bowl-shaped Metaldetes profundus (Billings) encrusted on one side by thromboid (t) and on the other side first by Girvanella (g), second by cryptic Archaeotrypa sp. (c), and third by Renalcis; right side of Metaldetes perforated by Trypanites macroboring (b); stick-shaped Metaldetes at right and Retilamina amourensis Debrenne and James at lower left and lower right; upper Botoman Forteau Formation (western Newfoundland, Canada). Collection of Pratt. B, Radiocyath Girphanovella georgensis (Rozanov) with branching archaeo-cyath Cambrocyathellus tuberculatus (Vologdin) and abundant pseudomorphosed aragonite botryoids; Atdabanian Salaany Gol Formation (Zuune Arts Mount, Zavkhan Province, Mongolia); MNHN Collection of Debrenne SM X 27,720.

settings (Astashkin et al. 1984; Kennard and James 1986; Kennard et al. 1989; Pratt 1995). Biomicrite interpreted to have been bound by mats, a common motif in middle Paleozoic deeper-water mud mounds, occurs in some Early Cambrian reefs ( James and Gravestock 1990).

Calcified Microbes

A wide variety of "microfossils" of accepted microbial origin is present in Cambrian reefs and indeed is integral to many frameworks. Calcification of filamentous cyano-bacteria in various stages of degradation formed tubules and threads referable to Gir-vanella and similar microfossils, and these are common as tabular crusts or subordinate encrustations (Kruse et al. 1996) (figure 12.1A). Discontinuous, planar to arcuate Girvanella crusts composed of tangles and multifilament sheets compose laminar zones up to a meter thick in archaeocyath-Renalcis reefs (Rees et al. 1989), and meter- to decameter-scale, conical to hemispheroidal tufa-like masses that are common in Middle and Late Cambrian platform-margin reefs and downslope mud mounds (James 1981; Kobluk 1985; Pratt 1989, 1995, 2000) (figure 12.2B).

Renalcis, Epiphyton, and similar objects are millimeter-sized aggregates of hollow and solid micritic spheroids and clots. In Early Cambrian reefs these formed freestanding masses that grew upward, as well as encrustations on cavity walls—especially the undersides and insides of archaeocyaths and Girvanella crusts, and within cavities excavated from matrix bioclastic lime mud (Kobluk and James 1979; James and Gravestock 1990; Wood et al. 1993; Kruse et al. 1995, 1996; Zhuravlev and Wood 1995) (figure 12.1A). In Middle and Late Cambrian reefs in both shelf and platform-margin settings, Renalcis is attached to thromboids and Girvanella crusts, and dendritic forms like Epiphyton are abundant in a pendent habit (James 1981; Pratt 1995: figure 7) (figure 12.2B).

Sponges

Sponges are represented by the extinct calcified class Archaeocyatha and the siliceous Hexactinellida and demosponge groups Anthaspidellidae and Axinellidae. The probable calcarean Gravestockia has also been reported from the Atdabanian low-energy biomicrite mud mounds of South Australia (Debrenne and Reitner, this volume: figure 14.1E).

Archaeocyaths are the most conspicuous and most abundant metazoan in Early Cambrian reefs, having contributed to patch reefs as old as earliest Tommotian (Riding and Zhuravlev 1995). Modular Archaeocyathida exhibit their greatest generic diversity in the Botoman (Wood et al. 1992b). Ajacicyathids and branching mono-cyathids formed thickets on soft substrates (figure 12.2A), and archaeocyathids attached to firm or hard substrates by means of aporous epitheca (Debrenne and

Figure 12.2 Thin-section photomicrographs of Cambrian frameworks. A, Branching mono-cyathid Archaeolynthus polaris (Vologdin) inter-growths with subordinate Renalcis, MNHN M810034, middle Tommotian Pestrotsvet Formation (Zhurinskiy Mys, middle Lena River, Siberia, Russia). B, Girvanella crusts with pendent Epiphyton; Upper Cambrian, Sunwaptan

Figure 12.2 Thin-section photomicrographs of Cambrian frameworks. A, Branching mono-cyathid Archaeolynthus polaris (Vologdin) inter-growths with subordinate Renalcis, MNHN M810034, middle Tommotian Pestrotsvet Formation (Zhurinskiy Mys, middle Lena River, Siberia, Russia). B, Girvanella crusts with pendent Epiphyton; Upper Cambrian, Sunwaptan

Cow Head Group (western Newfoundland, Canada). Collection of Pratt. C, Anthaspidellid demosponge Wilbernicyathus donegani Wilson encrusted from the top by Girvanella and eocrinoid holdfasts; Upper Cambrian, Sunwaptan Wilberns Formation, upper Morgan Creek Limestone (Llano Uplift, Texas, USA). Collection of Spincer.

Reitner, this volume: figures 14.3A and 14.4A). Their dominant orientation is sideways or downward growth in cavities (Zhuravlev and Wood 1995). This habit argues against the suggestion that archaeocyaths contained symbiotic photoautotrophs. The skeletons were originally high-Mg calcite (James and Klappa 1983), except for Dic-tyocyathus translucidus, which was aragonitic because it occurs consistently as blocky calcite cement-filled molds (Kruse et al. 1995).

Anthaspidellids with local encrustations of microbial filaments form the framework of late Middle Cambrian (late Marjumian) reefs from Iran (Hamdi et al. 1995; Zhuravlev et al. 1996; Debrenne and Reitner, this volume: figure 14.1B). In the Late Cambrian (Sunwaptan) of Texas, complete sponges are intergrown with thromboids and compose up to 25% of the boundstone (Spincer 1996) (see figure 12.2C). In some Early Cambrian reefs (James and Kobluk 1978; Kobluk and James 1979; Wood et al. 1993) and Late Cambrian (Sunwaptan) intrashelf thrombolite reefs (Pratt 1995: figure 11B), sediment-stabilizing hexactinellids left spicules and rare root tufts in the matrix.

Cnidaria

Modular, tabulate coral-like skeletons have been documented from Early Cambrian reefs. The Botoman of South Australia yields Flindersipora and some other tabulate-like corals (Lafuste et al. 1991; Sorauf and Savarese 1995; Debrenne and Reitner, this volume: figure 14.6D). Also, reefs of probable Botoman age from the Canadian Rocky Mountains and Labrador contain centimeter-sized remains of variably polygonal skeletons lacking septa (figure 12.1A). These have been described as Rosellatana, Ar-chaeotrypa, and Labyrinthus (Kobluk 1979,1984; Kobluk and James 1979). Their form of increase similar to longitudinal fission, however, does suggest affinities with chae-tetids (calcified sponges) rather than mainstream corals (Scrutton 1997).

Corals have not been observed in younger Cambrian reefs; tabulate corals reappear in the lowermost Ordovician reefs of western Newfoundland (Pratt and James 1982, 1989). More material is needed to ascertain whether or not the Late Cambrian (Marjumian) coral-like Cambrophyllum Fritz and Howell (1955) is found associated with reefs.

Problematica

Several sessile taxa restricted to Early Cambrian reefs contribute to frameworks but are of uncertain affinity and function. Cysticyathus from the middle to late Tommotian is a saclike fossil up to 3 cm in diameter, with porous walls and thin, widely spaced tabulae (Kruse et al. 1995). It has been suggested to be a "coralomorph" (Zhuravlev et al. 1993). The somewhat similar Tabulaconus from the Botoman of Cordilleran Lau-rentia bears many closely spaced arcuate and locally bifurcating tabulae, and it may too be a coral-like organism (Debrenne et al. 1987). The Hydroconozoa are represented by narrow to broad cones composed of lamellar calcite (Korde 1963), which have been regarded tentatively as "coralomorphs" (Zhuravlev et al. 1993). They are common encrusting elements usually found in cavities, in the Atdabanian and Botoman of Siberia and Mongolia (Wood et al. 1993).

Early Cambrian objects with a variably laminated structure have been placed in the Tannuolaiidae (=Khasaktiidae) and regarded as possible stromatoporoids (Sayutina 1980). Specifically, Khasaktia forms arcuate laminar encrustations a few millimeters thick on framework surfaces in the early Atdabanian of Siberia, and ramose Rackov-skia is found in the late Atdabanian to early Botoman of Mongolia, Siberia, and the Urals. However, domical encrustations much larger than Khasaktia occur in the probable Botoman of the Canadian Rocky Mountains, and these have a remnant microstructure and habit consistent with a high-modular, filter-feeding organism that may be a stromatoporoid-grade sponge (Pratt 1994).

Radiocyaths are irregularly conical structures up to several centimeters in size, composed of stout, weakly fused, originally aragonite structures (nesasters) similar to meromes of Receptaculitida (Zhuravlev 1986). Whole specimens occur worldwide from the late Tommotian to the Toyonian and are subordinate to archaeocyaths or form their own thickets (Zhuravlev and Sayutina 1985; Kennard 1991; Kruse 1991; Wood et al. 1993) (figure 12.1B).

Cribricyaths are 0.5 mm wide, undulating to irregularly coiled tubes that encrust the sides and undersides of archaeocyaths and Renalcis. They occur in the Atdabanian and Botoman of Siberia and Mongolia (Jankauskas 1972; Wood et al. 1993) and may have been filter feeders.

Wetheredella consists of small encrustations of coiled, overlapping micrite-walled tubes about 100 mm wide and has been observed rarely in cavities in the late Botoman of Labrador (Kobluk and James 1979). The subcircular cross section of the tubes suggests that this object may be a foraminifer rather than a calcified cyanobacterium (contra Kazmierczak and Kempe 1992). Wetheredella has not been reported from other Cambrian strata.

Synsedimentary Cementation

All reefs contain some proportion of CaCO3 cement precipitated on the sea floor, which enhanced rigidity. Cambrian reefs with large pores and growth-framework cavities exhibit abundant isopachous, acicular to bladed high-Mg calcite cement (James and Klappa 1983). Micritic calcite cement occurs in small intraskeletal, inter-particle, and fenestral pores. Early Cambrian reefs differ from younger Cambrian examples in that many contain botryoids of now-calcitized acicular aragonite up to several centimeters in size (James and Klappa 1983; Wood et al. 1993; figure 12.1B). Such cements from Mongolia have been interpreted as algae and named Zaganolomia

(Drozdova 1980). A temporal control may have been present, in that botryoidal cements have been documented from Nemakit-Daldynian to Botoman reefs but not in Middle and Late Cambrian reefs (Zhuravlev 1993).

REEF-DWELLING ORGANISMS Trilobites

Trilobites can usually be observed in the matrix of Cambrian reefs. They are essentially absent from Girvanella frameworks but present in surrounding facies. In Early Cambrian reefs they are typically scattered as small bioclasts less than a few millimeters in size. Reef-dwelling trilobites are generally believed to have constituted a community that differed from off-reef sites (Mikulic 1981), and this belief is supported by data specifically for the Cambrian (Repina 1983; Sukhov and Pegel' 1986). Throm-bolites from the Middle Cambrian (Marjumian) of the Siberian Platform and Late Cambrian (Sunwaptan) of the Canadian Rocky Mountains and Arctic Islands preferentially contain the centimeter-sized convex cephala and pygidia belonging to pletho-peltids (Stephen R. Westrop and Pratt, pers. obs.), perhaps a morphology adapted for a libero-sessile burrow-dwelling habit (Stitt 1976). Stigmacephaloides curvabilis is another taxon that appears restricted to reefs of Sunwaptan age (Spincer, pers. obs.). Reefal trilobites presumably grubbed on the sediment surface in search of organic particles and perhaps meiofauna.

Bivalved Arthropods

Cambrian bivalved crustaceans display a great deal of convergence, and many taxa are no longer regarded as true ostracodes (Hou et al. 1996). However, some Early Cambrian reefs contain disarticulated valves that possess a finely prismatic shell microstructure identical to that of younger ostracodes, which are also common bioclasts in reefs (Kobluk and James 1979).

Brachiopods

Lingulate brachiopods are rare in Cambrian reefs, and examples probably represent "stray" individuals, although Middle Ordovician mud mounds do host a diverse fauna specific to that setting (Krause and Rowell 1975). Calciate brachiopods were present in reefs since Atdabanian as disarticulated and articulated valves and include kutorgi-nids, obolellids (James and Klappa 1983; Ushatinskaya, this volume: figure 16.5), and orthids (Kruse 1991). In the Middle Cambrian, calciates (billingsellids) formed extensive shell beds, which served as substrate for anthaspidellid reefs and stromatolites (Zhuravlev et al. 1996), but they resumed a common reef-dwelling habit in the Early Ordovician.

Stenothecoids

The stenothecoid group embraces bivalved, brachiopod-like shells that include forms with and without hinge articulation. Their affinity is uncertain, and they are not distinctively molluscan (Yochelson 1969; Rozov 1984). They occur mostly in reefs of Atdabanian to Amgan age and were probably immobile, epifaunal suspension feeders (Spencer 1981; Kouchinsky, this volume).

Hyoliths

Centimeter-sized conical shells with an operculum, originally aragonite composition, and lenticular, triangular, trapezoidal, or subcircular cross sections have been considered mollusks (Marek and Yochelson 1976) or a separate phylum (Runnegar et al. 1975). Hyolithomorph hyoliths are common in matrix biomicrite as well as in non-reefal beds, reaching their maximum diversity in the Botoman (Rozanov and Zhuravlev 1992). They have not been reported to be associated with younger reefs. Their mode of life is unknown, but they may have been semisessile suspension feeders (Kruse et al. 1995; Kouchinsky, this volume). Orthothecimorph hyoliths were common in Nemakit-Daldynian and Tommotian peri-reefal grainstones and sometimes were immured in muddy bioherms as vertically oriented cones (Landing 1993). Thus, a suspension-feeding strategy may be implied for some of them.

Salterellids

Salterellids are another group with centimeter-sized conical shells but are composed largely of lamellar calcite ( James and Klappa 1983). Proposed as belonging to the phylum Agmata (Yochelson 1977), they appear to be restricted to the Laurentian Botoman and are also largely peri-reefal fossils.

Mollusks

Millimeter-sized helcionelloids occur in Tommotian to Atdabanian peri-reefal grain-stones (e.g., Moreno-Eiris 1987; Kruse et al. 1995), are relatively rare in reefs after the Sinsk event (James and Klappa 1983), and appear to be absent in reefs of the younger Cambrian. The similar-sized gastropod Sinuella is abundant in Sunwaptan reefs of Texas, although it appears rarely in correlative rocks elsewhere in Laurentia.

Echinoderms

Echinoderm ossicles are present in reefs from the latest Atdabanian, whereas they may form grainstones in associated beds (Kobluk and James 1979; Rozanov and Zhuravlev 1992). Their precise taxonomic affinity is often uncertain, owing to the rarity of articulated specimens. Eocrinoid ossicles are common in late Middle and Late Cam brian thrombolites, and holdfasts are observed contributing to frameworks (Spincer 1996; Zhuravlev et al. 1996) (figure 12.2C).

Chancelloriids

Disarticulated, hollow, star-shaped sclerites belonging to chancelloriids are abundant in Early Cambrian reefs and peri-reefal sediments (James and Klappa 1983) but have been observed only in nonreefal facies in the Middle Cambrian. Soft-bodied preservation in the Early and Middle Cambrian shows that chancelloriids may have been sessile animals covered in sclerites (Briggs et al. 1994:212-213) and were probably not sponges (Mehl 1996).

Small Shelly Fossils

Various microfossils difficult to assign on the basis of petrographic characteristics occur in Early Cambrian reefs as old as early Tommotian (Riding and Zhuravlev 1995; Kruse et al. 1995) and as young as Atdabanian (Wood et al. 1993). These fossils are more common in nonreefal beds. Such bioclasts are rare after the middle Botoman (after the Sinsk event) and are absent in younger Cambrian reefs.

Boring Organisms

The macroboring ichnogenus Trypanites, whose holes are similar to those produced by sipunculid worms, occurs abundantly on the upper surfaces of some Early Cambrian reefs in Labrador (James et al. 1977). However, its rarity on substrates within reefs (James and Gravestock 1990; Kruse 1991) (figure 12.1A) suggests that the borer was only occasionally part of the reef-dwelling community proper. Macroborings have not been described within younger Cambrian reefs.

Endolithic "algal" microborings have been documented from ooids beginning in the Vendian (Green et al. 1988), but these are rare in Cambrian reef-associated bio-clasts (Kobluk and Kahle 1978; Conway Morris and Bengtson 1994). Pervasively mi-critized shell material, characteristic of Cretaceous and Cenozoic endolithic infestation, is not present. Silt-size microspar grains resembling "chips" from clionid-type sponge boring have been identified by Kobluk (1981), and possible sponge borings may occur in Botoman coral-like skeletons from the Canadian Rocky Mountains (Pratt 1994). Scalloped surfaces are locally present and are suggestive of rasping activity (Zhuravlev and Wood 1995).

Burrowing Organisms

Bioturbation is present in all Cambrian reefs, including those of Nemakit-Daldynian age (Zhuravlev and Wood 1995; Kruse et al. 1996), that possess muddy sediment around and within the framework, although repeated phases of total reworking do not seem to be typical. Burrows are discrete, unbranched, meandering, millimeter-wide tubes that commonly have infilling microspar or micrite and, locally, pellets; the concave lamination from deliberate backfilling, as seen in many burrows in siliciclastic facies, is absent. Sub-millimeter-sized burrows are locally branching and typically partly empty. Burrow style suggests a certain firmness of the sediment by the time the observed burrow generations were made.

Miscellaneous Microorganisms

Reef sediments undoubtedly hosted biodegrading bacteria, and the presence of pyrite indicates sulfate-reducing bacteria also. Kobluk and James (1979) reported possible fungi as filaments and as tangled "fecal pellets," which could represent fungal infestation after pellet formation. The fungal interpretation is not yet convincing, however, and similar hematitic filaments in red Late Devonian mud mounds have been interpreted as bacterial (Bourque and Boulvain 1993). Zhuravlev and Wood (1995) illustrated presumably calcified filaments 0.3-1 mm wide that are encrusting the underside of an archaeocyath, and they interpreted them as fungi. The fungal identification of these structures has yet to be confirmed.

COMMUNITIES Environmental Setting

Cambrian reefs occupied depositional settings in tropical to subtropical, normal marine waters at the shelf-slope break along the margins of carbonate platforms or shelves, in the middle of platforms or near shore in shallow water, as well as in deeper water downslope or in intrashelf basins (James and Kobluk 1978; Astashkin 1981; Kennard et al. 1989; Pratt 1989, 2000; James and Gravestock 1990). The variety of energy levels is mirrored in the kinds of flanking sediments, which may be ooidal and oncoidal, or thin, nodular-bedded argillaceous lime mudstone. The proportion of mi-crite and biomicrite decreases with increasing energy, but there may be other factors involved in the sediment-generating capacity of the reef.

Although filter-feeding archaeocyaths indicate the presence of abundant suspended organic material and sufficient nutrients, the dominance of microbial structures in all Cambrian reefs points to the overall clarity of the water. Regional profiles may show lateral variation in framework type (figure 12.3), but the reasons for this are not easy to decipher. Early Cambrian reef-bearing units are typically intercalated with purely siliciclastic units, and reefs in nearshore areas commonly have admixed subangular silt and fine sand. The ecological tolerance of archaeocyaths is difficult to assess, but their filter-feeding capacity and skeletal construction suggest that they were comparatively

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