干旱区生态环境知识资源中心

Middle Miocene to Pliocene siliciclastics of the Bare Formation represent a long-lived (ca. 11 Myr) break in the otherwise carbonate-dominated shelf of the Northern Carnarvon Basin, Northwest Shelf of Australia. The quartz-sandstone interval is correlated with the appearance of spectacular clinoform sets mapped on 3D and dense 2D seismic data. Twenty-seven clinoform sets are interpreted as delta lobes primarily based on their plan-view morphology (strike-elongate to lobate features) and their 40-100-m-high clinoform amplitudes. The delta lobes were deposited on outer-shelf to shelf-edge positions, and the older deltas show evidence of a higher degree wave reworking than the younger deltas. Measurements of the along-strike (migration) and down-dip (progradation) movement of these deltas are compared with relative sea-level behaviour inferred from shelf-edge trajectory analysis. Delta lobes exhibit greater lateral shifting during relative sea-level rise, whereas delta lobes are more restricted to dip-oriented fairways during sea-level fall, although no major incised valleys have been identified. Long-term (cumulative) progradation of this delta system and subsequent backstepping correlates with long-term sea-level fall and rise during the late middle and late Miocene. In addition, a long-term northeastward migration trend for these delta lobes was likely a result of localized uplift of an inversion anticline in the RosemaryLegendre Trend; the growth of this anticline probably steered the fluvial source for the delta system towards the northeast. The Bare Formation siliciclastic influx correlates with other middle Miocene increases in siliciclastic sediment supply worldwide. Global cooling and a shift to more arid conditions, negatively influencing vegetation cover, may have combined with more seasonally variable rainfall to generate the high sediment supply that built the deltas. Retreat of the siliciclastics could correlate with ice-sheet growth in the Northern Hemisphere and/or increase in the Indonesian Throughflow and Leeuwin Current (ca. 1.6 Ma), which might have modified climate regionally.