Development of spatially diverse and complex dune-field patterns: Gran Desierto Dune Field, Sonora, Mexico

The pattern of dunes within the Gran Desierto of Sonora, Mexico, is both spatially diverse and complex. Identification of the pattern components from remote‐sensing images, combined with statistical analysis of their measured parameters demonstrate that the composite pattern consists of separate populations of simple dune patterns. Age‐bracketing by optically stimulated luminescence (OSL) indicates that the simple patterns represent relatively short‐lived aeolian constructional events since ～25 ka. The simple dune patterns consist of: (i) late Pleistocene relict linear dunes; (ii) degraded crescentic dunes formed at ～12 ka; (iii) early Holocene western crescentic dunes; (iv) eastern crescentic dunes emplaced at ～7 ka; and (v) star dunes formed during the last 3 ka. Recognition of the simple patterns and their ages allows for the geomorphic backstripping of the composite pattern. Palaeowind reconstructions, based upon the rule of gross bedform‐normal transport, are largely in agreement with regional proxy data. The sediment state over time for the Gran Desierto is one in which the sediment supply for aeolian constructional events is derived from previously stored sediment (Ancestral Colorado River sediment), and contemporaneous influx from the lower Colorado River valley and coastal influx from the Bahia del Adair inlet. Aeolian constructional events are triggered by climatic shifts to greater aridity, changes in the wind regime, and the development of a sediment supply. The rate of geomorphic change within the Gran Desierto is significantly greater than the rate of subsidence and burial of the accumulation surface upon which it rests.     

Nature and composition of interbedded marine basaltic pumice in the ∼52–50 Ma Vastan lignite sequence,western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels – one at ～5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ～52–50 Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated altered basaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO ₂, Al ₂ O ₃, TiO ₂, MgO, alkalies and REE, but very high in Fe ₂ O ₃, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO ₂, Al ₂ O ₃, MgO and variable gain in Fe ₂ O ₃, TiO ₂, Ni, V, Zr, Zn and REE. Bubble initiation to growth in the ascending basaltic magma (liquidus ～1200–1250 ^°C) may have occured in ～3 hr. Short-distance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.