Tuesday, June 24, 2014

A Look Back…25 Years: Mixed Dolomite

The “dolomite problem”—how great thicknesses of dolomitized platform carbonates form—has vexed geologists for decades. In this paper, Humphrey and Quinn described the nature and distribution of dolomite from three late Pleistocene raised reef terraces in southeastern Barbados, West Indies. The data facilitated a conceptual model that was then numerically tested by computer simulation. The results revealed the how interaction of sea level fluctuations, sedimentation, rapid recurring dolomitization, and subsidence controlled generation of dolomitic successions.  

Coastal mixing zone dolomite, forward modeling, and massive dolomitization of platform-margin carbonates by John D. Humphrey and Terrence M. Quinn, Journal of Sedimentary Petrology, v. 59, p. 438-454.

Thursday, June 12, 2014

A Look Back…10 Years: Global Ocean Chemistry from Echinoderms

Secular changes in ocean chemistry are related to the mineralogy of carbonate precipitates, and had been predicted by global ocean models.  Ten years ago, Dickson described the Mg content of Cambrian to Eocene echinoderm ossicles and compared the data to other first-order geochemical cycles and proxies. The data revealed systematic changes coincident with previous interpretations of ocean Mg/Ca ratios, with high mole% MgCO3 in Early Cambrian and late Carboniferous to Triassic samples, and low values in Silurian and Jurassic to Cretaceous echinoderms. The paper suggested that the data “add another independent line of evidence that collectively can leave little doubt that major changes in the seawater Mg/Ca ratio have occurred,” but noted other short-term changes.

EchinodermSkeletal Preservation: Calcite-Aragonite Seas and the Mg/Ca Ratio ofPhanerozoic Oceans by J.A.D. (Tony) Dickson, Journal of Sedimentary Research, v. 74, p. 355-365.

Wednesday, June 4, 2014

Highlights—Calling Out Megaclasts

Although the expanded Udden-Wentworth (U-W) grain-size scale is an important tool for classifying the size of sedimentary particles, existing terminology at the coarse end of the scale is problematic. In this paper, Terry and Goff review how several terms used to define size ranges on the U-W scale also have shape applications on particle form diagrams, which can lead to confusion. To resolve this issue, the paper proposes abandoning shape-related terms and replacing them with a new incremental system based on meso- and macro- prefixes to classify large boulder size ranges. The value of the modification is that both size and shape classification of very coarse sediments can now be accomplished simultaneously without overlap or confusion in nomenclature.