ORIGIN OF SUBMARINE PISOLITHS AND THE SEDIMENTOLOGY OF MIDWESTERN SILURIAN REEFS

Vertical tension fractures that crosscut proximal flank beds exposed i n the Pipe Creek Junior Quarry in north-central Indiana formed during compaction of underlying interreefal carbonate mud, and are commonly f illed with well-sorted pisolitic grainstone made up of coated grains r anging up to 1 cm in diameter. Pisolith cortices are petrologically in distinguishable from marine cement between pisoliths, from marine ceme nt lining tension fractures, and from marine cement lining vugs in sur rounding flank beds. Isotopic signatures for these components are also consistent (delta(13)C = -0.6 to 2.3 parts per thousand PDB; delta(18 )O = -8.7 to -3.9 parts per thousand PDB). In addition, pisolites are present exclusively within tension fractures, and commonly show latera l size grading, with grain size increasing toward fracture interiors. These features show that pisoliths are in fact mobile marine cements t hat precipitated from Silurian seawater in synsedimentary tension crac ks that formed soon after deposition of flank beds. Requisite conditio ns for the formation of such large coated grains include high-energy, shallow-water settings, where sea level is generally coincident with t he depositional surface of the reefal complex. This origin supports an interpretation that allochthonous material forming flank beds was pre dominantly generated immediately upslope on wave-swept platforms, that in situ accumulation of biotic debris was insignificant in the format ion of the Pipe Creek Junior complex, and that many of the larger Silu rian ''reefal'' buildups like Pipe Creek Junior throughout the midwest ern United States probably had a similar origin.