In the Neogene Siwalik molasse of the Himalayan foreland basin, sandstones are replaced vertically by conglomerates over roughly a hundred metres, with an approximately 100-fold jump in median grain size (D50D_{50}). Dubille and Lavé (2015) showed that this stratigraphic gravel-sand transition (GST) mimics the abrupt transition from gravel-bed to sand-bed channels observed today on the Gangetic Plain 8-20 km downstream of the range front, and proposed that the stratigraphic transition records the progressive southward migration of a hydraulic sorting front (a migrant gravel front) driven by continuous thrust-wedge advance, implying that the transition is diachronous by nature rather than the signal of a tectonic or climatic event. Later work has quantified migrating conglomerate-sandstone transitions in other orogens and measured the abrasion, recycling, subsidence, and episodic-flood processes that set gravel runout, yet for the Himalayan source system itself no basin-scale, along-strike chronology of GST ages exists at sufficient resolution to quantify lateral diachronism and discriminate steady gravel-front migration from episodic tectonic or climatic forcing, nor to test whether the preserved thickness and abruptness of the transition are consistent with long-term average migration rates. The open problem is this basin-scale test of the genesis of the Himalayan GST.

Background

Foreland basins form where the load of a growing mountain belt flexes the lithosphere, creating a subsiding trough that fills with sediment delivered from the rising range; the classic terrestrial fill of such a basin is called molasse. Along the Himalayan southern margin, the Neogene to Quaternary fluvial fill of the foreland basin is exposed in the Siwalik hills, forming the Siwalik Group, conventionally divided into Lower (fine sandstone and mudstone), Middle (thick sandstone), and Upper (sandstone overlain by coarse conglomerate) subunits. Across much of the Himalaya the passage from Middle Siwalik sandstone to Upper Siwalik conglomerate is strikingly abrupt: gravel deposits replace sand in vertical succession over about a hundred metres, and the median grain size (D50D_{50}, the diameter below which half the sediment mass is finer) increases by a factor of roughly 100. Radical grain-size changes of this kind in foreland megacycles have traditionally been read as records of changes in tectonic activity or climate in the adjacent mountains.

In modern rivers worldwide, a gravel-sand transition (GST) occurs where gravel-bed channels give way to sand-bed channels over only a few kilometres, with an order-of-magnitude drop in bed grain size and gradient. In central Nepal this transition lies 8-20 km downstream of where rivers exit the Siwalik Range onto the Gangetic Plain, and the D50D_{50} ratio between the gravel-bed and sand-bed reaches is about 100, the same factor as the stratigraphic jump. The dominant explanation for GSTs is selective deposition: as a river loses transport capacity, it deposits its coarsest load first, so the bed fines rapidly downstream without any change in water or sediment supply. Dubille and Lavé (2015) argued that the Siwalik sandstone-conglomerate transition is the stratigraphic record of exactly this process: as the Himalayan thrust wedge (the package of rock deformed by forward-propagating thrusts, whose current leading edge is the Main Frontal Thrust) advanced southward into its own basin, the alluvial gravel front, meaning the basinward limit of gravel deposition set by selective sorting during downstream runout of the coarse bedload, migrated southward with it, and each depositional site eventually recorded the crossing of this sorting transition as an abrupt vertical coarsening. A key corollary is that the GST must be diachronous, that is, different along-strike sections should record the sandstone-to-conglomerate switch at different times. In a steady-state orogen-foreland system the gravel front migrates with the advancing thrust wedge, so the lateral offset of the facies limit and the thickness of the preserved transition are linked through the front-advance (shortening) rate and the sediment-accumulation (aggradation) rate at the depositing site, and both rates may vary along strike. The authors supported the picture with a coarse mass balance for gravel leaving the fold-thrust belt, and with evidence for quartz enrichment of the conglomerates by differential weathering, clast abrasion, and recycling of earlier foreland deposits during folding and exhumation of the Siwalik hills, and they showed that piedmont drainage reorganizations and piggyback basins (basins carried passively atop an active thrust sheet) can complicate the record locally.

Testing the migrant-gravel-front hypothesis at basin scale requires dating the GST in many sections distributed along the roughly 2000 km of Himalayan strike. The relevant chronological tools are magnetostratigraphy (correlating the recorded sequence of geomagnetic polarity reversals in a section to the geomagnetic polarity timescale) and cosmogenic burial dating (using the decay of cosmogenic nuclides such as 10^{10}Be accumulated in quartz before burial to date sediment deposition), which for Siwalik sections currently yield age models with uncertainties of order 0.5-1 million years in places. Later work has identified the obstacles that remain: sparse and uncertain GST ages; episodic rather than steady facies migration caused by discrete fold activation and piggyback-basin formation; reworking of channel sediments for thousands of years before final burial under low subsidence rates, which blurs how faithfully short-term GST-position variability is locked into long-term stratigraphy; and only crude constraints on the coarse-bed gravel flux leaving the range and on clast survival fractions across transport cycles, which are needed to check that gravel runout can indeed generate the observed transition. Since 2015, substantial progress has been made on the process side in both the Himalaya and comparable orogens, but the central basin-scale test in the source system is still outstanding.

Problem Statement

Test the migrant-gravel-front hypothesis for the Himalayan sandstone-conglomerate transition at the scale of the foreland basin, as left open by Dubille and Lavé (2015). Specifically: (1) establish the age of the gravel-sand transition (GST) in chronostratigraphically dated Siwalik sections distributed along the Himalayan front, that is, in sections spread along the strike rather than in one or two localities, with uncertainties small enough to resolve the along-strike age differences sought in (2), improving on the ±0.5-1 Myr uncertainties of many existing magnetostratigraphic and cosmogenic-burial age models; (2) quantify the degree and spatial pattern of along-strike lateral diachronism of the GST, restoring sections to their depositional positions relative to the thrust front; and (3) decide from that pattern whether the transition records progressive southward migration of a hydraulic gravel front driven by thrust-wedge advance under the facies-migration plus selective-sorting mechanism, as expected at long-term average rates in a steady-state orogen-foreland system and allowing the migration rate to vary along strike, or whether synchronous tectonic or climatic forcing, discrete fold activation, piggyback-basin capture, or drainage reorganization dominate the stratigraphic expression, assessing in that comparison whether the restored along-strike age pattern and the preserved thickness and apparent abruptness of the transition are consistent with such long-term average migration. Because the preserved transition integrates episodic facies migration and wedge widening, thousands-of-years-scale channel reworking before burial under low subsidence rates, and intermittent coarse-gravel bypass past the transition (for example by hyperconcentrated floods), it does not automatically mark a stable time line, and episodic jumps are among the alternative controls on the preserved geometry that the discrimination in (3) must take into account. The overarching objective is a quantitative, basin-scale account of how the abrupt Siwalik sandstone-conglomerate transition formed and migrated; each deliverable above is a linked ingredient of that single account, not an independent target. The task is resolved only by that joint account, not by any one ingredient on its own. Ingredient (1) is met by critically compiling and reassessing the existing chronostratigraphic constraints on the GST (the magnetostratigraphic and cosmogenic-burial age models, together with any additional published dates) so as to sharpen the GST ages; generating new field geochronology is not required, provided the resulting ages carry uncertainties small enough to resolve the along-strike age differences sought in (2). Basin scale means GST ages from at least six dated sections distributed across the roughly 2000 km of Himalayan strike in more than one along-strike segment rather than in one or two localities, each restored to its depositional position relative to the thrust front; on that basis (2) is met by a quantified along-strike age pattern with stated uncertainties, and (3) by a mechanism discrimination that also tests the preserved thickness and apparent abruptness of the transition against long-term average migration rates.

References

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