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.
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.
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.
