Differential Diagenesis of Rhythmic Limestone Alternations Supported by Palynological Evidence

Abstract

Alternating cemented and uncemented, fine-grained layers from Pliocene periplatform carbonates of the Great Bahama Bank have fundamentally different diagenetic features. The cemented layers consist predominantly of microspar, interpreted as an early marine, shallow-burial cement. The intercalated, uncemented, softer layers are devoid of microspar cement and exhibit signs of mechanical compac- tion. Precursor sediments of both cemented and uncemented layers apparently consisted largely of aragonite needles. The needles in the compacted layers show signs of dissolution, suggesting that the calcium carbonate required for cementation of the uncompacted layers was provided by aragonite dissolution within the compacted layers. The lack of compaction in the cemented layers shows that cementation, and hence aragonite dissolution in the adjacent compacted layers, took place in the shallow-burial realm. The dissolved carbonate was trans- ported by diffusion to adjacent layers and reprecipitated as calcite cement, thereby preventing significant compaction of these layers. These processes are not yet complete in the material examined, and some aragonite remains in the compacted layers. The sedimentary composition of the two rock types (compacted and uncompacted) is similar, indicating a similar precursor sediment for both. Between 1.5 and 7 times the concentration of palynomorphs oc- curs in the compacted layers, apparently the result of passive diage- netic enrichment. Although the trigger for diagenetic differentiation has not been determined, the pure limestone succession studied here appears to serve as a clay-free analog to limestone-marl alternations.