Thrusting and gravel progradation in foreland basins: A test of post-thrusting gravel dispersal

The use of gravels as syntectonic indicators of thrusting has recently been questioned by foreland-basin models that assign gravels to a post-thrusting interval of progradation, except in very proximal areas. On the basis of precise temporal control provided by magnetostratigraphically dated sections, the history of gravel progradation after a major thrusting and uplift event in the northwestern Himalaya is shown to be a virtually syntectonic phenomenon. Despite considerable crustal subsidence driven by a thick-skinned thrust, gravels prograded 7 0 km during a 1.5-m.y.-long thrusting event. By 3 m.y. after the start of thrusting, gravels extended more than 110 km into the basin. Although delayed gravel progradation appears appropriate for many Rocky Mountain foreland basins, it is clearly not valid for the Himalaya. We attribute the difference in depositional response between these basins to differences in the quantity of sediment supplied to them (sediment starved vs. overfilled), the availability of resistates in the source area, and the size of the antecedent drainage. INTRODUCTION Reconstructions of the absolute and relative timing of thrusting events along the margins of foreland basins have frequently been based on interpretations of the stratigraphic record. Progradation of conglomeratic facies from a thrustbounded source terrain has often been cited as an indicator of an episode of thrusting (e.g., Armstrong and Oriel, 1965; Wiltschko and Dorr, 1983; Heller et al., 1986). In the proximal regions of foreland basins where conglomerates may be in intimate association with adjacent thrusts (e.g., Riba, 1976), it is reasonable to tie conglomeratic packages, especially syntectonically deformed ones, to thrusting events. In many basins, however, subsequent deformation and erosion have removed this very proximal record of thrusting, and so the most proximal strata preserved may be tens of kilometres from their source terrains. Because facies progradation is time transgressive, the ages of influx of a conglomeratic sequence will vary between locations, and none will provide a precise age for the causative thrusting event (Jordan et al., 1988); therefore, other approaches to dating thrust activity must be considered. In foreland basins where very precise time constraints are available on facies migration, such as in northern Pakistan (Raynolds and Johnson, 1985), it may be possible to extrapolate an observed rate of gravel progradation backward in time and space toward the thrust front in order to estimate the time of initial progradation (Burbank and Raynolds, 1988). The accuracy of such an estimate is dependent on the frequently untestable assumption of a steady rate of progradation through time. When a thrust elevates a new, lithologically distinct source terrain, sandstone compositions across a broad part of the foreland basin may record this change almost immediately (Jordan et al., 1988). Analyses of provenance variations are only useful the first time a new lithologic component is introduced by thrusting and therefore are not applicable to many thrusted foreland sequences that repeatedly expose lithologically similar source areas. The concept of conglomeratic facies as reliable indicators of thrusting events has recently been questioned. Beck (1985) pointed out that in some of the Rocky Mountain foreland basins, thrusting was associated with subsidenceinduced, fine-grained sedimentation in much of the proximal part of the basin, except immediately adjacent to the thrust shelf itself. Significant parts of the distal foreland were dominated by fluvial systems flowing toward the thrust front, and lacustrine facies occasionally spread across the medial part of the basin (Beck, 1985; Beck et al., 1988). The conglomeratic sheets that prograded across the basins and that had formerly been interpreted as indicators of thrusting events (Wiltschko and Dorr, 1983; Armstrong and Oriel, 1965) were described as primarily postorogenic facies related to cessation of thrusting (Beck and Vondra, 1985). These interpretations of Cretaceous and Tertiary sedimentation in the Rockies have provided a basis for conceptual models that attempt to link thrusting, crustal flexure, subsidence histories, and facies geometries in foreland basins (Beck et al., 1988; Kvale and Beck, 1985; Heller et al., 1988; Blair and Bilodeau, 1988). According to these models, thrust loading induces a predominantly elastic crustal response in which the maximum basin subsidence is localized adjacent to the thrust. Because the rate of subsidence frequently outpaces the rate of gravel supply, the coarse-grained facies derived from the uplifted terrain are confined to a narrow belt adjacent to the thrust. The depositional axis of the basin is displaced toward the thrust belt and is often dominated by fine-grained sedimentation. Moreover, the major tributaries to the foredeep are rivers flowing toward the zone of maximum subsidence from the distal margin. Consequently, throughout much of the foreland basin, episodes of thrusting will be recorded by either low-energy axial deposition or by rivers flowing toward the thrust belt. When thrusting ceases and erosion reduces the thrust load, these models predict that thrust-induced subsidence will slow or cease and conglomeratic facies will prograde extensively across the basin (Beck and Vondra, 1985; Kvale and Beck, 1985; Heller et al., 1988). Furthermore, Heller et al. (1988) suggested that, after a thrusting event, erosion of the thrust sheet will cause isostatic rebound, generating widespread proximal unconformities. Can these proposed interpretations of conglomeratic facies and their relation to thrusting be tested against existing data sets? In order to evaluate the model rigorously, stratigraphic data with high-resolution time control and also independent evidence of the initiation and cessation of thrust activity are needed. This paper describes such a data set from the northwestern Himalaya—probably the most precisely dated terrestrial foreland basin succession in the world. Interpretation of the local deformational and depositional history suggests that the models for delayed gravel progradation are not applicable to the thick-skinned thrusts in the northwest Himalaya. GEOLOGY, v. 16, p. 1143-1146, December 1988 1143 on January 30, 2015 geology.gsapubs.org Downloaded from