PALEOCLIMATIC CONTROLS ON DETRITAL MODES OF JURASSIC AND CRETACEOUS SANDSTONES, CRATONIC MARGIN, WESTERN INTERIOR BASIN

by
G.A. Ludvigson, B.J. Witzke, R.R. Anderson, and R.M. McKay

The Geological Society of America
30th Annual North-Central Section Meeting
Iowa State University, Ames, IA May 2-3, 1996
1996 Abstracts with Programs, v. 28, no. 6, p. 52

ABSTRACT

(Late?) Jurassic (approx. 160 Ma) and mid-Cretaceous (approx. 100 Ma) sandstones in western Iowa were deposited on the eastern, cratonic margin of the Mesozoic North American Western Interior Basin, and were derived from the westward fluvial transport of material eroded from Paleozoic sedimentary and Precambrian basement rocks in the upper midwest. Detrital modes in the two populations are dramatically different. Jurassic sandstones are feldspathic calclithites (mean compositions are Q₃₇F₄L₅₉; Qp₁₀Lss₂Lsc₈₈; and Q₄₁Lss₁Lsc₅₇; n = 7), whereas Cretaceous sandstones are ultramature quartzarenites and quartz wackes (mean compositions are Q₉₈F₀L₂; Qp₉₃Lss₇Lsc₀; Q₉₉Lss₁Lsc₀; n = 12). Significant volumes of clastic material in each population were derived from Paleozoic carbonate rock terranes, as shown by the abundance of chert clasts (grouped in Qp) in each. Sedimentary carbonate rock fragments (Lsc) are a major constituent in Jurassic sandstones, whereas they are completely lacking in the Cretaceous sandstones. These observations are compatible with other lines of evidence that show that Jurassic sandstones accumulated in an arid environment (with interbedded sulfate evaporites), whereas the Cretaceous sandstones record intense chemical weathering in a humid environment. Factors controlling this climatic shift include: (1) During the 160-100 Ma interval, northward continental drift carried the Iowa area from a drybelt (~ 30°N) into humid temperate mid latitudes; (2) onset of the mid-Cretaceous "greenhouse world" contributed to accelerated chemical weathering of cratonic regions worldwide; and (3) general circulation model experiments suggest that mid-Cretaceous global average precipitation was substantially higher than present (Barron et al., 1989, Global and Planetary Change, v.1, p. 157-174).