Browsing by Subject "Sedimentary processes"
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Item Controls on sedimentary processes and 3D stratigraphic architecture of a mid-Miocene to recent, mixed carbonate-siliciclastic continental margin : northwest shelf of Australia(2011-05) Sanchez, Carla Maria, 1978-; Fulthorpe, Craig Stephen, 1954-; Steel, R. J.; Mohrig, David; Austin, James A.; Kerans, Charles; Janson, XavierDetermining the relative importance of processes that control the generation and preservation of continental margin stratigraphy is fundamental to deciphering the history of geologic, climatic and oceanographic forcing imprinted on their sedimentary record. The Northern Carnarvon Basin (NCB) of the North West Shelf of Australia has been a site of passive margin sedimentation throughout the Neogene. Cool-water carbonate sedimentation dominated during the early-middle Miocene, quartz-rich siliciclastics prograded over the shelf during the late-middle Miocene, and carbonate sedimentation resumed in the Pliocene. Middle Miocene to Pliocene siliciclastics were deposited as clinoform sets interpreted as delta lobes primarily based on their plan-view morphology and their relief of 40-100 m. Shelf-edge trajectory analysis suggests that part of this stratigraphic succession was built during a long-term, third order, regressive phase, producing shelf-edge deltas, followed by an aggradational episode. These trends appear to correlate with third-order global eustatic cycles. Slope incisions were already conspicuous on the slope before deltas reached the shelf-break. Nevertheless, slope gullies immediately downdip from the shelf-edge deltas are wider and deeper (>1 km wide, ~100 m deep) than coeval incisions that are laterally displaced from the deltaic depocenter (~0.7 km wide, ~25 m deep). This change in gully morphology is likely the result of greater erosion by sediment gravity flows sourced from shelf-edge deltas. Total late-middle to late Miocene margin progradation increased almost three times from 13 km in the southwest to 34 km in the northeast, where shelf-edge deltas were concentrated. Flat-topped carbonate platforms seem to have initiated on subtle antecedent topographic highs resulting from these deltaic lobes. A reduction of siliciclastic supply to the outer paleo-shelf during the Pliocene combined with the onset of a southwestward-flowing, warm-water Leeuwin Current (LC) most likely controlled the initiation of these carbonate platforms. These platforms display marked asymmetry, likely caused by an ancestral LC, which created higher-angle, upcurrent platform margins, and lower-angle, downcurrent clinoforms. The along-strike long-term migration trend of the platforms could be the result of differential subsidence. These platforms constitute the first widespread accumulation of photozoan carbonates in the Northern Carnarvon Basin. They became extinct after the mid-Pleistocene when the LC weakened or became more seasonal.Item Mechanisms of Sedimentation Inferred from Quantitative Characteristics of Heavy and Light Minerals Sorting and Abundance(2014-07-16) Motanated, KannipaHydraulic behaviors of particles with contrasting sizes and densities and flow structure of hyperconcentrated suspensions were empirically studied in a thin vessel by using particle-image-velocimetry techniques. Particle volumetric concentration, C_(v), of the suspensions were mainly affected by the majority particle species, silica ballotini. The minority population was aluminum ballotini (hydraulically fine). At high C_(v), aluminum particles were less retarded and settled as if they were hydraulically coarser. This was because particles moved in a cluster-like motion. Terminal settling velocities of both particles converged at C_(v )? 25%, and particle sorting was diminished. Spatial and size distributions of mineral grains with contrasting densities in massive sandstones of turbidites from the Middle Permian Brushy Canyon Formation were used to estimate suspended sediment concentrations and interpret hydraulic evolution of the turbidity currents. Semi-quantitative elemental distributions were estimated by x-ray fluorescence analytical microscopy, ?XRF. Within the structureless sandstone, zircon grains (hydraulically fine) fined upward while feldspar grain (hydraulically coarse) sizes did not change. Both grains had hydraulically equivalent settling velocities in overlying siltstone layers. These suggest that these sandstone divisions were deposited from hyperconcentrated suspensions where particle segregation was diminished and hydraulically fine grains were entrained with hydraulically coarse particles. While structureless sandstones were deposited, C_(v) increased through distance and time because hydraulically fine particles were fining upward. This evolution likely be resulted from volumetric collapse of the turbidity currents. Geochemical concentrations and properties of Zr- and Ti-rich particles were used to qualitatively estimate erosional event sizes of hemipelagic thinly laminated siltstone. Zircon and rutilated quartz particle sizes, Zr/Ti fluorescence ratio, and lamination thickness were determined by ?XRF. Zircon grains were finer than rutilated quartz grains. Their grain sizes were systematically correlated but neither was correlated with Zr/Ti ratio. Instead, Zr/Ti ratio covaried with lamination thickness. Since zircon grains are smaller but heavier than rutilated quartz grains, zircon has lower susceptibility to erosion, particularly by wind. Thus, fluctuations of Zr/Ti fluorescence ratio in Brushy canyon Formation siltstones most likely result from variations in the intensity of erosional events at the particle source or sources, with high Zr/Ti ratios reflecting periods of intense erosion.