Browsing by Subject "Sea level"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Contribution of additional sediment to a basin by valley incision and controls on valley preservation in stratigraphy(2015-05) Pace, Allison Jonell; Fisher, W. L. (William Lawrence), 1932-; Mohrig, DavidIn stratigraphy, there have been many studies looking at the geomorphological features generated during sea level variations and the resulting preserved erosional composite unconformities that evolve over time and thus do not correlate to a valley that existed at one given time (Strong & Paola, 2008). Data from the XES 02 experiment that isolated sea level variations from sedimentation and subsidence rates offered a way to look at the exclusive influence of sea level fluctuations on sediment transportation and stratigraphic preservation of valleys (Kim et al., 2006a). This study estimated the amounts of sediment deposited in an experimental basin through time in order to determine (1) the role of valley incision as a basinal sediment source, , and (2) how much of a valley formed during maximum sea level fall is preserved within the basin stratigraphy. Significant valley incision was found during times of rapid sea level fall. Valley incision increased erosion rates up to 5 times those recorded during sea level highs, resulting in a doubling of the basinward sedimentation rates. Stratigraphic preservation of valleys was low, but in those locations where a valley fragment was observed, most of the original valley wall remained. More instances of valley preservation occurred when rapid sea level falls occurred during times of slow, overall sea level rise. Maximum reworking and basinward transport of sediment occurred during rapid sea level falls in an overall falling sea level regime, while preservation of the valleys was greatest during times when rapid falls occurred in an overall rising sea level regime.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 Processes and architectures of deltas in shelf-break and ramp platforms : examples from the Eocene of West Spitsbergen (Norway), the Pliocene paleo-Orinoco Delta (SE Trinidad), and the Cretaceous Western Interior Seaway (S. Wyoming & NE Utah)(2008-12) Uroza, Carlos Alberto, 1966-; Steel, R. J.This research investigates different scenarios of deltaic deposition, both in shelfbreak and ramp settings. I address four ancient cases with particular characteristics: 1) A shelf-margin case from the Eocene Battfjellet Formation, West Spitsbergen, Norway, in which deltas were able to migrate to the shelf-edge during rising and sea-level highstand conditions despite the low-supply character of the system (low progradation/aggradation rates compared to analogous margins), with consequent sand starvation on the slope and deeper areas of the basin. The delta system was overall wave-dominated, with restricted tide-influence at the mouth of the distributaries and more accentuated tide-influence during the transgressive transit of the deltas; 2) A shelf-margin case from the Pliocene paleo-Orinoco Delta System, Mayaro Formation, SE-Trinidad, in which high rates of sediment supply from the paleo-Orinoco River and exceptionally high subsidence rates due to growth-faulting, produced a spectacular stacking of sandstones on the outer shelf and shelf-edge areas, but with apparently limited sand delivery into deeper waters. The delta system was overall storm-wave dominated, with fluvial-influence in the lower segment of the system and some tide-influence in association with the fluvial-influence; 3) A case from a shallow-water ramp, Campanian Rock Springs Formation (Western Interior Seaway), in which deltas accumulated along relatively straight, north-south oriented shorelines highly impacted by wave-storm processes. Tide-influence was limited to the mouth of the distributaries, and fluvial deposits mostly developed within the coastal-plain areas; and 4) A case from the same ramp setting as (3) but in an outer-ramp site, Campanian Haystack Mountains Formation, in which a lowering in sea-level translated the delta system tens of kilometers eastwards into the basin. As a consequence of a shallower and narrower seaway, southerly-oriented tidal currents were enhanced and subsequently skewed or re-aligned the delta system to the south. The key contributions of this research concern (1) the feasibility of shelf-margin accretion during rising and highstand of sea level, (2) the critical importance of shelf width and sediment supply (and not only sea-level behavior) to bring deltas to the shelfedge, and (3) the possible tendency for tides enhancement in the distal reaches of shallow seaway ramps, caused by narrowing of the seaway and fault-topography enhancement during falling sea level.