Controls on distribution, timing, and evolution of turbidite systems in tectonically active settings: The Cretaceous Tres Pasos Formation, southern Chile, and the Holocene Santa Monica Basin, California.

摘要

Deep-marine basins along active continental margins are commonly the ultimate site of deposition for sediment that originated from adjacent eroding highlands. Preserved deep-marine strata from these settings are typically the most complete record of sediment transfer from continents to oceans over geologic time scales. The research summarized in this dissertation investigates spatial and temporal patterns of deep-marine sedimentation in such tectonically active areas. The overarching goal is to evaluate controls on sedimentation within a broader context that considers both external drivers to the system as well as intrinsic complexity. Toward that end, the separate chapters in this dissertation span a broad range of scales. At the smallest scale, single sedimentation units are analyzed to understand processes of deposition. The characterization of multiple sedimentation units that compose mappable sedimentary bodies is important for deciphering the geomorphic conditions at deposition and is the crucial link to a larger stratigraphic understanding. Finally, at the largest scale, fundamental controls on sedimentation related to basin-scale configuration and evolution of sediment source area are addressed within a plate tectonics context. A range of temporal scales and resolutions is also addressed in this research through the examination of both ancient (Chapters 1 and 2) and modern (Chapter 3) deep-marine systems.; Chapter 1 summarizes a detailed outcrop study of exceptional exposures of the Upper Cretaceous Tres Pasos Formation at Cerro Divisadero, southern Chile. Four sandstone-rich packages (10-40 m thick) are separated by comparably thick intervals of shale and silt and mapped across a continuous 2.5 km-long transect parallel to depositional dip. The lower sandstone units record deposition from weakly confined to unconfined sand-laden flows, are associated with mud-rich mass transport deposits, and interpreted to represent a base-of-slope to lower slope setting. The upper sandstone units exhibit evidence of increasingly more channelization and net bypass indicative of higher gradient positions on the slope. This is interpreted as a record of the basinward progradation of the delta-slope. The scale of the delta-fed slope system represented by these strata is comparable in size to large continental margin-scale systems and is thus important for our general understanding of how continental margins evolve.; Chapter 2 addresses larger-scale questions regarding the tectonic evolution of the foreland basin in which the Tres Pasos and overlying Dorotea formations accumulated. Results from >450 new detrital-zircon ages show that the incorporation of Upper Jurassic igneous rocks (ca. 147-155 Ma) into the Andean fold-thrust belt and their associated denudation was established by the Campanian (ca. 80 Ma) and was a significant source of detritus by the Maastrichtian (ca. 70 Ma). Sandstone compositional trends indicate an increase in volcanic and volcaniclastic grains associated with this unroofing pattern. Chapter 3 investigates the history and timing of coarse-grained sediment input into a modern deep-marine basin. This study utilizes the distribution and timing of turbidite deposits in Santa Monica Basin, offshore southern California, to evaluate controls on sediment flux to the basin over the past 7,000 years. Six new radiocarbon dates are integrated with five previously published, but recalibrated, dates from a 12.5 meter-thick turbidite section from Ocean Drilling Program (ODP) Site 1015 in the basin plain. An increase in sediment delivery to the basin starting ∼2 ka is a function of three interacting factors: (1) increase in magnitude and frequency of El Nino-Southern Oscillation (ENSO) cycles ∼2 ka that led to increased flux of the Santa Clara River, (2) change in routing of coarse-grained sediment within the staging area from direct river input to indirect, littoral cell input at ∼3 ka, and (3) decreasing rates of s