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Fine-grained rocks of Late Cretaceous, West Coast, active margin strata have generally been ignored in paleoenvironmental analyses because of the highly fragmented and apparently homogeneous nature of outcrops. Recent studies on Holz Shale (Ladd Formation, Santa Ana Mountains) fine-grained slope strata have shown that diagenetic carbonate concretions which occur in this unit are useful for understanding primary sediment fabric and hence paleoenvironments. Usefulness of concretions to such paleoenvironmental studies can only be evaluated, however, after their diagenetic history is fully understood.
Holz Shale concretions most commonly occur as ellipsoids 0.05-1.5 m (0.2-5.0 ft) in diameter. Concretions generally consist of amorphous shale clay and quartz sediment cemented by calcite. Organic materials such as mollusk valves and terrestrial plant material commonly served as concretion nuclei. On the basis of this association with preserved organic materials and the abundance of pyrite preserved within concretions, it appears highly probable that decomposition of organic materials by sulphate-reducing bacteria was an important factor in the formation of these concretions. Lack of compaction of trace fossils within concretions, bending of strata around concretions, presence of septarian structures, and penecontemporaneous slumping of concretions in surrounding sediments indicate an early diagenetic origin where original sediment fabrics were preserved.
Many other Late Cretaceous, deep-marine, active margin, fine-grained strata on the West Coast contain similar concretions. If these concretions prove to have an early diagenetic origin like those in the Holz Shale, they may be the key to a better understanding of depositional mechanisms of these widespread deposits.
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