ABSTRACT

Vertical tension fractures that crosscut proximal flank beds exposed in the Pipe Creek Junior Quarry in north-central Indiana formed during compaction of underlying interreefal carbonate mud, and are commonly filled with well-sorted pisolitic grainstone made up of coated grains ranging up to 1 cm in diameter. Pisolith cortices are petrologically indistinguishable from marine cement between pisoliths, from marine cement lining tension fractures, and from marine cement lining rugs in surrounding flank beds. Isotopic signatures for these components are also consistent (¹³C = -0.6 to 2.3 PDB; ¹⁸O = -8.7 to - .9 PDB). In addition, pisolites are present exclusively within tension fractures, and commonly show lateral size grading, with grain size increasing toward fracture interiors.

These features show that pisoliths are in fact mobile marine cements that precipitated from Silurian seawater in synsedimentary tension cracks that formed soon after deposition of flank beds. Requisite conditions for the formation of such large coated grains include high-energy, shallow-water settings, where sea level is generally coincident with the depositional surface of the reefal complex. This origin supports an interpretation that allochthonous material forming flank beds was predominantly generated immediately upslope on wave-swept platforms, that in situ accumulation of biotic debris was insignificant in the formation of the Pipe Creek Junior complex, and that many of the larger Silurian "reefal" buildups like Pipe Creek Junior throughout the midwestern United States pr bably had a similar origin.