PHANEROZOIC PALEOGEOLOGY

Current geochemical cycling models do not account for changes in the spatial distribution of rock exposed to weathering through time. As a step in this direction, a new method of constructing paleogeologic maps has been developed, using published, global, depositional lithofacies maps. This technique identifies areas of erosion for a given interval of time, then systematically searches back through lithofacies maps of progressively older periods to determine the most probable type of rock being weathered during the interval. The age-area results for present-day sedimentary exposures show the same lognormal relationship previously determined by Blatt and Jones (1975). A geologic map of the Recent generated by this method reproduces the general igneous, marine, and continental clastic spatial and temporal distributions found in current outcrops. However, the total exposure area of sandstones is larger than previous estimates based on volume, not areal proportions. We propose that exposed sandstone deposits are extensive, but thin (and thus represent a much smaller volume than areal proportion of sedimentary rocks). The relative proportions of exposed rock types through Phanerozoic time show few abrupt changes compared to the secular changes in the areal extent of the depositional lithologies. The global distribution of exposed rock types through time suggests that exposed cratonic shield areas have generally been replaced in extent with clastic sediments, but this trend may be an artifact of the mapping technique. For paleoclimate modeling, these determinations of paleogeologic distribution should ultimately provide a great advantage over parameterizations based on global estimates of sedimentary reservoirs.