Silurian continental distributions, paleogeography, climatology, and biogeography

Abstract

Continental orientations during the Silurian Period have been determined using paleoclimatic in addition to paleomagnetic data. The influence of climate on lithology is particularly marked during periods like the Silurian when epeiric seas were widespread and sedimentation was dominantly autochthonous (evaporites, carbonates, reefs, authigenic minerals) and therefore reflective of climate at the depositional site. During such times, with few large land areas in low latitudes, one would expect climatic patterns to have been more zonal than cellular, and also that long river systems (capable of transporting clastic sediments from wet to dry belts) would not have existed. Therefore, even allochthonous deposits, particularly thick sequences of coarse elastics can be added to the list of paleoclimatic indicators.Silurian northern hemisphere atmospheric circulation can be modeled on present patterns in the southern hemisphere because of the lack of significant land influence on climate. The wet-hot (10[deg]N--10[deg]S), dry-warm (10[deg]--30[deg]), wet-cool (30[deg]-60[deg]), dry-cold (60[deg]--pole) pattern is amply confirmed by Silurian sediment distribution on those paleocontinents whose orientations have been established from paleomagnetic measurements (Laurentia, Baltica, Siberia). Paleozoic sedimentation in these zones is as follows: 10[deg]N--10[deg]S, thick elastics and reefs associated with leading plate margins, and carbonates and reefs in the epeiric seas; 10[deg]--30[deg], evaporites, carbonates and reefs; 30[deg]--60[deg], clastics, coals and tillites; 60[deg]--pole mostly tillites. The other paleo-continents (Kazakhstania, North China, South China, Gondwana) can be oriented by using the above lithologic associations in ways consistent with their known convergent and collision patterns in the late Paleozoic. All were in relatively low latitudes with the exception of Gondwana which was over the South Pole. A large north polar ocean existed which must have had an ameliorating effect on climate in the northern hemisphere, while the opposite was true of the southern hemisphere.The conclusion that most paleocontinents had extensive epeiric seas and were positioned in low latitudes accounts for the cosmopolitan nature of Silurian faunas. Only Gondwana in the south (Clarkeia fauna), and Mongolia in the north (Tuvaella fauna) shows signs of provincialism and low faunal diversities. This situation can be contrasted with the Devonian, when the collision of Laurentia and Baltica resulted in land barriers and marked provincialism.