Investigation of a >6-km-thick succession of Cretaceous to Cenozoic sedimentary rocks in the Tajik Basin reveals that this depocentre consists of three stacked basin systems that are interpreted to reflect different mechanisms of subsidence associated with tectonics in the Pamir Mountains: a Lower to mid-Cretaceous succession, an Upper Cretaceous-Lower Eocene succession and an Eocene-Neogene succession. The Lower to mid-Cretaceous succession consists of fluvial deposits that were primarily derived from the Triassic Karakul-Mazar subduction-accretion complex in the northern Pamir. This succession is characterized by a convex-up (accelerating) subsidence curve, thickens towards the Pamir and is interpreted as a retroarc foreland basin system associated with northward subduction of Tethyan oceanic lithosphere. The Upper Cretaceous to early Eocene succession consists of fine-grained, marginal marine and sabkha deposits. The succession is characterized by a concave-up subsidence curve. Regionally extensive limestone beds in the succession are consistent with late stage thermal relaxation and relative sea-level rise following lithospheric extension, potentially in response to Tethyan slab rollback/foundering. The Upper Cretaceous-early Eocene succession is capped by a middle Eocene to early Oligocene (ca. 50-30 Ma) disconformity, which is interpreted to record the passage of a flexural forebulge. The disconformity is represented by a depositional hiatus, which is 10-30 Myr younger than estimates for the initiation of India-Asia collision and overlaps in age with the start of prograde metamorphism recorded in the Pamir gneiss domes. Overlying the disconformity, a >4-km-thick upper Eocene-Neogene succession displays a classic, coarsening upward unroofing sequence characterized by accelerating subsidence, which is interpreted as a retro-foreland basin associated with crustal thickening of the Pamir during India-Asia collision. Thus, the Tajik Basin provides an example of a long-lived composite basin in a retrowedge position that displays a sensitivity to plate margin processes. Subsidence, sediment accumulation and basin-forming mechanisms are influenced by subduction dynamics, including periods of slab-shallowing and retreat. K E Y W O R D S basin subsidence, foreland basins, geodynamics, stratigraphy, subduction-related basins, tectonics and sedimentation 526 | EAGE CHAPMAN et Al.

No abstract

No abstract

This page was generated automatically upon download from the ETH Zurich Research Collection. For more information, please consult the Terms of use.

Microfossil distribution patterns and high-resolution δ 13 C and δ 18 O curves, calibrated against planktonic foraminiferal and calcareous nannofossil data, are provided for the Aptian pelagic Coppitella section of the Gargano Promontory (southern Italy). The succession consists of cyclically arranged couplets of bioturbated grey marlstones and off-white marly limestones, referable to the Marne a Fucoidi. In the lower portion of the section, two thin black shales were recognized. The highresolution δ 13 C curve presented here correlates with those of other Alpine-Tethyan sections, albeit with lower absolute values. The onset of deposition of organic-rich sediments falls at the top of the interval of unchanging carbon-isotope values, whereas the upper black shale is documented from the interval of the main Aptian positive δ 13 C excursion. According to our biostratigraphic data, the deposition of organic matter in the Gargano Promontory persisted through Early/Late Aptian boundary time. Using a chemostratigraphic definition, only the lower black shale is referred to the Selli Level. As far as the biotic response is concerned, the onset of the 'nannoconid crisis' is recorded considerably below the lower black shale, whereas the 'Globigerinelloides eclipse' is recorded below and within the upper black shale. The distribution of meso-eutrophic indices (Zygodiscus spp., radiolaria) vs. moderate-fertility indices (Rhagodiscus asper and Lithraphidites carniolensis) testifies to a modest increase of surfacewater fertility only throughout the stratigraphically higher black shale. The occurrence of a benthic foraminiferal fauna, albeit impoverished, in both the basal and upper black horizons clearly documents dysaerobic rather than completely anoxic conditions on the sea floor. Relative sea-level rise at the time of the Selli Event in the Gargano Promontory is documented by drowning and foundering of the Apulia platform margin, situated adjacent to the basin in which the Marne a Fucoidi accumulated. ‡Author for correspondence: Hugh.Jenkyns@earth.ox.ac.uk

Summary Magneto-stratigraphic and nannofossil assemblage data from ODP Site 1165 evidence an anomalous warming event of the surface waters in and around Prydz Bay during the Early Pleistocene. This results from an increase in the abundance of nannofossils at Site 1165, that occurred at 1 Ma. High-resolution sampling permits a new bio-magnetostratigraphic interpretation for ODP Site 1165. A decrease in δ18 O values at Sites 1165 and 1167 also occurs at this time, supporting the presence of warming conditions in the Prydz Bay area. A return to colder surface waters, indicated by the absence or rare occurrence of nannofossils in the upper cores from Site 1165, suggests that more stable glacial conditions existed in the Prydz Bay basin for the last 900 ka. These new evidences call for a reevaluation of the notion that the East Antarctic Ice Sheet has experienced stable conditions similar to today since the late Neogene.

No abstract

No abstract