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 prob- ably the largest ever recorded in a thrombolite reef. Four main types of components are distinguished in the reef frame- work: (1) dense bushy mesoclots, (2) dense rounded to subrounded mesoclots, (3) saccate microstructures, and (4) crusts, mostly struc- tureless, 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 re- mains of what were initially either aggregates of the probable filamen- tous cyanobacterum Angulocellularia or aggregates of renalcid calci- microbes, or both, and the saccate mesoclots as the calcimicrobe Ren- alcis 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 protome- soclots 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 rap- id deposition including graded bedding, poor sorting of bioclasts, and textural inversion are common in internal sediment in meter-size cav- ities. These features suggest that storms, possibly hurricanes, were at least as important, and probably more so, than ''everyday'' wave en- ergy in transporting sediment into all cavities in the reef, regardless of size.