SANDS OF TIME: THE CHANGING DISTRIBUTION OF NEARSHORE ARENITES THROUGH THE PHANEROZOIC

Sedimentary rock volume has been shown to be a first-order control on fossil distribution through time. In addition to total rock volume, the shifting representation of sedimentary facies may be an important factor in controlling Phanerozoic diversity patterns. In order to assess the potential impact of uneven facies representation we compiled a global database of nearshore sands throughout the Phanerozoic. This facies is an ideal test case because it has an easily recognized suite of sedimentary structures and a characteristic ichnofauna. Importantly, unlike other well-studied facies such as shallow sub-tidal carbonates or coals, the distribution of nearshore sands is not climate-controlled.

The distribution of nearshore sands through the Phanerozoic is strongly bimodal with peaks in the early Paleozoic and late Mesozoic-Cenozoic: 37 % of all named units are of either Cambrian or Cretaceous age, while only 7% are of Permian or Triassic age. This pattern, which persists when data are normalized to interval duration and to total rock volume, reflects changes in the total perimeter of coastlines through time due to formation and breakup of supercontinents, and related eustatic fluctuations. Although the database is global in scope, it is heavily biased towards Laurentia –more than 2/3 of the units are North American or European. Though Gondwana appears to show a similar trend, data are insufficient.

Although they rarely contain well-preserved body fossils, the vertical tube-like trace fossil Skolithos is closely associated with nearshore sands throughout the Phanerozoic. As expected given the distribution of nearshore sands, the distribution of Skolithos occurrences is also strongly bimodal. This study supports previous work demonstrating non-random distribution of sedimentary facies through time, and shows that such there is significant variation even in the distribution of sediments which are independent of climate and latitudinal controls.