Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States

Abstract

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU’s FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC01448, Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States, Christian J. Bjerrum and Rebecca J. Dorsey). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $15.00. Payment must accompany order.A thick succession of Jurassic nonmarine and marine sedimentary rocks is exposed in a large area from northern Arizona to eastern Idaho and western Wyoming. These sediments accumulated in the Utah‐Idaho trough, a deep elongate cratonal basin whose origin has recently been debated. Detailed stratigraphic analysis, subsidence analysis, and first‐order flexural modeling of these deposits (this study) provide new insights into the timing and mechanisms of subsidence in the Utah‐Idaho trough. Lower and Middle Jurassic strata are divided into six unconformity‐bounded sequences. In contrast to the overall uniform thickness of Lower Jurassic sequences (1 and 2), Middle Jurassic strata (sequences 3 through 6) consist of distinctly westward thickening sedimentary packages in which basal shallow marine deposits onlap eastward onto bounding unconformities. Basal strata of sequences 3 through 6 pass upward into widespread progradational continental deposits that are truncated progressively toward the east (cratonward) by the next unconformity. Decompacted total subsidence curves show that the rate of subsidence in most sections increased sharply at the end of sequence 2 time (J‐2 unconformity; ∼170 Ma). This is interpreted to record the onset of Middle Jurassic deposition in the distal part of a retroarc foreland basin. The unconformities and distinctive stratal geometries may have formed in response to forebulge migration caused by episodic thrusting in the Cordilleran orogen to the west. First‐order flexural modeling was carried out to test the hypothesis of flexural subsidence in the Utah‐Idaho trough. Trial‐and‐error comparisons produce a close match between decompacted stratigraphic profiles and model deflection profiles. The best fit is obtained using an infinite elastic plate (D = 1 × 1024 Nm), a moderate load topography, elevated base level, and an overfilled basin. Using recently published tectonic reconstructions for Nevada and Utah, we find close spatial agreement between a large Middle Jurassic fold‐thrust belt and the supracrustal load inferred from model simulations. Our integrated basin analysis thus supports the interpretation of some previous studies that the Middle Jurassic Utah‐Idaho trough was a retroarc foreland basin that formed east of a belt of regional contractile deformation and crustal thickening in western and central Nevada and southeastern California. Late Jurassic extension and normal faulting in northeastern Nevada may have been related to gravitational collapse of overthickened crust in the Cordilleran orogen. This coincides with a period of slowed subsidence in the Utah‐Idaho trough that began at about 157 Ma.