March GEOLOGY and GSA TODAY media highlights

The nature of shallow-water carbonate lithofacies thickness distributions

Peter Burgess, Shell International E&P, EPT-RXF, PO Box 60, Rijswijk, Rijswijk 2280 AB, Netherlands. Pages 235-238.

Carbonate lithofacies thickness distributions are of fundamental importance to understanding shallow-water carbonate deposystems because they record evidence of the lateral distribution and migration of lithofacies elements, as well as evidence for the various other intrinsic and extrinsic controls on stratal geometries and accumulation rates. Previous analyses of lithofacies thickness data led to the suggestion that exponential distributions are ubiquitous in the ancient record. This has been interpreted to indicate deposition by stochastic Poisson process lithofacies mosaics. To further investigate these ideas, Burgess performed a statistical analysis of 56 outcrop and core examples. The Kolmogorov-Smirnov test was used to identify the degree to which measured lithofacies thicknesses are well represented by a theoretical exponential distribution. Results from this analysis show that 16 of the 56 examples can be confidently shown to be exponential, while 28 are probably not exponential. This indicates that stochastic Poisson processes are a plausible explanation for many carbonate successions, but they do not explain all of those tested here, suggesting that other non-Poisson processes, either stochastic or deterministic in nature, or both, must also be important. Thus lithofacies planform geometries, and the processes controlling vertical stacking in ancient carbonate platform top deposystems, were likely more diverse than has been suggested, requiring significant further quantitative analysis and numerical forward modeling.

Creation of a continent recorded in zircon zoning

Desmond Moser et al., Biology and Geological Sciences Building, Room 1070, Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada. Pages 239-242.

Moser et al. discovered ancient microcrystals, about as wide as a human hair, that grew over a period of 200 million years. The approximately 3-billion-year-old crystals of the mineral zircon were found in a metamorphosed bedrock outcrop of originally sedimentary rock in northern Ontario, Canada, where the lower crust has been pushed to the surface from depths of 30 kilometers. Like trees in old growth forests, the crystals were found to have roughly circular growth zones that Moser et al. were able to date and analyze with specialized ion probes. From core to rim, the zones track the formation of the early North American continent from its beginning as a series of volcanic island chains, to its eventual fusion into a large, thick continental plate. While crystals of this age were previously known, the longevity of the crystal growth that has been detected is truly remarkable and holds the promise that other crystal micro-records tracking planet evolution exist on Earth and rocky planets including Mars.

Tectonic burial and “young” (<10 Ma) exhumation in the southern Apennines fold-and-thrust belt (Italy)