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.