Abstract

Many ancient cool-water carbonates include little or no evidence for aragonite being present in their primary sediment composition, and consequently that they inherited a low diagenetic potential. However, despite differences in skeletal assemblages, age, and depositional settings, Plio-Pleistocene cool-water limestones from eastern North Island of New Zealand are characterized by locally abundant aragonite remains (typically c. 20%, range < 5% to > 60%), either directly as variably altered skeletons or indirectly as biomolds, oversized pores, and detrital material resulting from collapse of dissolved shells. Specifically, five types of aragonite preservation are evident: (a) skeletons with original aragonite; (b) skeletons with original aragonite but with leaching cavities; (c) skeletons with aragonite relics encased in either dirty or pale brown neomorphic calcite spar; (d) skeletons neomorphically altered to calcite preserving relic shell structures; and (e) dissolved-out skeletons leaving an empty or calcite-cement-filled cavity. All these types of preservation, including transitions between them, can occur within the same sample.

These observations motivate a conceptual model for the timing and complex pathways of neomorphism in these Plio-Pleistocene cool-water limestones, related to their diagenetic evolution from shallow marine to burial to uplift-related meteoric environments. In this, the aragonite alteration pathways suggest that the different neomorphic fabrics formed at any stage during the diagenetic history of the host carbonates and are likely not unequivocal indicators of specific diagenetic environments. Instead, differing alteration pathways can potentially produce similar fabrics. The observations are consistent with the interpretation that shell microstructures controlling the nature of the intraskeletal neomorphic front and the degree of openness of the diagenetic system were of greater importance in determining the neomorphic fabrics and relic preservation of aragonite than was the general diagenetic environment, be it marine, meteoric, or burial. Also important were external controls linked to the types of depositional settings that dictated the composition of carbonate factories, the morpho-tectonic evolution of the host basin, and the influence of relative sea-level fluctuations on sediment production, accumulation, and condensation. These results illustrate that factors controlling ultimate preservation of carbonate biofacies are multifarious and complex. Moreover, they suggest that the importance of aragonitic components could be underestimated for many ancient cool-water carbonate deposits, thereby compromising estimation of original skeletal assemblages, depositional environments, and sediment budgets.