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 thickenin...

Limits on extreme eolian systems: Sahara of Mauritania and Jurassic Navajo Sandstone examples

“…Although the literature shows a disagreement about the timing of the onset of foreland basin loading and asymmetric basin subsidence (Bjerrum and Dorsey, 1996;Allen et al, 2000), the pronounced east-to-west thickening of the Navajo and related units supports the subsidence-curve models of Allen et al (2000), in which a Lower Jurassic (Navajo) initiation of loading events in the Cordilleran orogen is given. Until regional unconformities are documented within the Navajo, it must be assumed that sediment influx, accumulation, and burial of the accumulations continued uninterrupted.…”

“…7). This eastward shift of the shoreline within this tectonically active basin seems best accredited to an eastward migration of the forebulge (c.f., Bjerrum and Dorsey, 1996).…”

“…Until regional unconformities are documented within the Navajo, it must be assumed that sediment influx, accumulation, and burial of the accumulations continued uninterrupted. Loss of preservation space within the basin occurs with regional uplift and creation of the J-1 and J-2 unconformities that truncate the Navajo and are interpreted as the response to the migration of the forebulge as a result of episodic thrusting in the Cordilleran Belt (Bjerrum and Dorsey, 1996). The forebulge, located east of the Capitol Reef area in the Allen et al (2000) model, appears to divide the Navajo sand sea into a well-developed dry system to the west and a more wet system with fluvial influx to the east (Verlander, 1995).…”

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“…Royse (1993) hypothesized that a Late Jurassic-Early Cretaceous foredeep that contained as much as 6 km of synorogenic sediment existed in western Utah, and was then completely eroded away during Cretaceous thrusting. Bjerrum and Dorsey (1995) and Allen et al (2000) proposed that Middle Jurassic sedimentary units exposed in Utah represent distal fl exural foreland basin sedimentation in response to crustal thickening in western Nevada and southeast California, and thus that eastern and central Nevada was the site of a Jurassic proximal foredeep. DeCelles and Currie (1996) and DeCelles (2004) proposed that Cordilleran foreland basin sedimentation began with accumulation of Late Jurassic strata in a back-bulge setting in Utah, and interpreted a Jurassic foredeep setting in eastern Nevada.…”

“…The contoured exhumation values should be interpreted as minima, because several studies have suggested that a mid-Mesozoic sedimentary section, possibly several kilometers thick, may have been deposited in eastern Nevada and western Utah, and subsequently eroded during Cretaceous thrust-related uplift (Royse, 1993;Bjerrum and Dorsey, 1995;Allen et al, 2000;DeCelles and Currie, 1996). Royse (1993) hypothesized that a Late Jurassic-Early Cretaceous foredeep that contained as much as 6 km of synorogenic sediment existed in western Utah, and was then completely eroded away during Cretaceous thrusting.…”