Biosedimentology of a Silurian Thrombolite Reef with Meter-Scale Growth Framework Cavities

Abstract

A completely dolomitized Silurian reef at Maumee, Ohio, U.S.A., is uncommon among early Paleozoic reefs in that its framework consists of about 95% thrombolite boundstone and 5% corals. The reef lithified syndepositonally, as evidenced by centimeter-to meter-scale growth framework cavities within it that are completely filled with internal sediment. The largest cavities, 2-3.5 m in diameter, are probably the largest ever recorded in a thrombolite reef.Four main types of components are distinguished in the reef framework: (1) dense bushy mesoclots, (2) dense rounded to subrounded mesoclots, (3) saccate microstructures, and (4) crusts, mostly structureless, but also containing dense bushy mesoclots. The dense bushy components are abundant; rounded to sub-rounded mesoclots less so, and the remaining components are rare. The dense bushy and rounded to subrounded mesoclots are interpreted to represent the calcified remains of what were initially either aggregates of the probable filamentous cyanobacterum Angulocellularia or aggregates of renalcid calcimicrobes, or both, and the saccate mesoclots as the calcimicrobe Renalcis sensu stricto. It is speculated that syndepositonal lithification of the reef framework was accomplished by impregnation of unlithified protomesoclots by micrite to create mesoclots along with the formation of micritic cement around and between mesoclots that served to fuse together successive generations of mesoclots.The direction of branching of bushy mesoclots in the reef indicates that unlithified protomesoclots were able to grow upward, laterally, and downward. This ability, along with syndepositional lithification to create mesoclots and to cement them together, enabled the protomesoclots and mesoclots to create reef knobs and, eventually, to roof over spaces that were centimeters to three or more meters in width and height to create growth framework cavities. Features indicative of rapid deposition including graded bedding, poor sorting of bioclasts, and textural inversion are common in internal sediment in meter-size cavities. These features suggest that storms, possibly hurricanes, were at least as important, and probably more so, than ''everyday'' wave energy in transporting sediment into all cavities in the reef, regardless of size.

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“…Sixty samples of deformation structures were examined. Dolomitization has not destroyed all traces of original textures in this unit, and sedimentary structures are readily recognizable (Kahle 1988(Kahle , 1994(Kahle , 2001.…”

Origin of Meter-Scale Submarine Cavities and Herringbone Calcite Cement in a Cambrian Microbial Reef, Ledger Formation (U.S.A.)

“…By comparison to modern settings (Boardman and Bergstrand 1993), the intercalation of the Ledger reef facies with ooid shoals indicates a highenergy shelf-margin depositional setting. Pratt (1989) and Kahle (2001) report similar microbial deposits of Late Cambrian and Silurian ages, respectively. The deposits are interpreted as having formed as reefs on shallow ramps or shelves (Pratt 1989;Kahle 2001).…”

“…Pratt (1989) and Kahle (2001) report similar microbial deposits of Late Cambrian and Silurian ages, respectively. The deposits are interpreted as having formed as reefs on shallow ramps or shelves (Pratt 1989;Kahle 2001).…”

Depositional Environment and Lithostratigraphy of the Paleoproterozoic Mooidraai Formation, Kalahari Manganese Field, South Africa

“…Thrombolitic bioherms and biostromes have been reported in stratigraphic successions of varying age in which thrombolites (or thrombolite-stromatolite associations) constitute the only microbial buildups within a carbonate succession (e.g. Aitken, 1967;Kennard and James, 1986;Riding et al, 1991;Feldmann and McKenzie, 1998;Grotzinger et al, 2000;Turner et al, 2000;Kahle, 2001).…”