Doctoral Dissertations

Date of Award

8-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Steven G. Driese, Kenneth R. Walker

Committee Members

Claudia I. Mora, Thomas W. Broadhead, Craig E. Barnes

Abstract

The Morrison Formation accumulated in the broad, shallow back-bulge depozone (Western Interior) of the Cordilleran foreland basin system during the Late Jurassic (?Oxfordian/Kimmeridgian - Tithonian). In east-central Colorado, the alluvial plain deposits of the Morrison Formation are composed of a complex sequence of interbedded floodplain, fluvial, and lacustrine deposits with minor evaporite and marine deposits. Within this alluvial plain complex, a well-developed lacustrine succession is present, which consists of two distinct lacustrine facies associations - an open and a marginal lacustrine facies association. The open-lacustrine facies association is composed of lacustrine carbonates (skeletal mudstone-wackestone and microbialites) and laminated mudstone. The marginal-lacustrine facies association consists of palustrine carbonates (carbonaceous packstone, intraclast wackestone-grainstone, peloid skeletal mudstone-wackestone, peloid skeletal packstone-grainstone, and micritic mudstone), ooid packstone-grainstone, green mudstone, siltstone, and sandstone. The paucity of well-laminated and organic-rich deposits indicates the water column of Morrison paleolakes was well-oxygenated, reflecting holomictic to polymictic conditions. Facies and stratigraphic relationships suggest that Morrison paleolakes were characterized by a central open-lake settings that graded laterally and vertically into marginal-lacustrine and eventually into adjacent distal-alluvial environments. The open- and marginal-lacustrine facies associations display evidence for frequent subaerial exposure and modification resulting from pedogenic activity, pseudomicrokarst, and meteoric diagenesis. The abundance of these features indicates that Morrison lakes are best characterized by low-gradient ('ramp'-type) margins that were subjected to periodic reworking by wind-driven currents, by storms and flood events, and where oscillations in lake level resulted in the exposure of large areas. High groundwater tables discharging into paleotopographic lows, with minor meteoric contributions, served as the primary water source for Morrison lakes. A diverse biota was present in the carbonate lakes of the Morrison paleoecosystem. Charophytes, ostracodes, prosobranch and pulmonate gastropods, and unionid bivalves were commonly observed, with lesser amounts of spongillids, microbialites, and conchostracans. Bone fragments indicate fish, crocodiles, and dinosaurs also were present within and around the lakes and ponds. Trace fossils record the presence of insects, crayfish, plants, and dinosaurs within and along the margins of Morrison lakes and ponds. Wind-driven currents and periodic high-energy events as well as scavenging/oxidation of organic matter were the primary taphonomic processes acting on the fossil assemblages. The lacustrine carbonate deposits are dominated by disturbed and transported fossil assemblages, with minor mixed and in-place fossil assemblages. Paleontological data indicate Morrison lakes were primarily freshwater and were characterized by perennial to ephemeral, and hydrologically open to closed conditions. Sedimentologic evidence indicates that a wide range of hydroperiods (15-325 days) characterized the lake settings. The major factors controlling carbonate sedimentation were the low clastic input, due to the distal alluvial setting and the presence of a clastic-filtering fringe of macrophytes along lake margins, and the abundance of a prolific carbonate-producing biota (i.e., charophytes). The results of stable isotope geochemistry revealed a lack of strong covariant trends (r = 0.1 to 0.50), which indicates that Morrison lakes were primarily hydrologically open. Isotopically-depleted soil CO2 due to pedogenic modification produced the heterogeneous δ13C values associated with the marginal-lacustrine (i.e., palustrine) deposits. The abundance and type of pseudomicrokarst associated with the lacustrine carbonate succession indicates the Late Jurassic climate was characterized by a distinct wet and dry season that was intermediate between semi-arid and sub-humid conditions. Magadi-type cherts and minor evaporites indicate intervals within the lacustrine succession when the climate temporarily deteriorated to more semi-arid conditions and the water column was more alkaline-saline. However, the lack of evaporites capping lacustrine sequences suggests the lakes maintained primarily freshwater, and probably hydrologically-open, conditions during carbonate precipitation. Stable isotope compositions also point toward non-evaporated lake waters, at least during carbonate deposition, due to the lack of covariant trends. The evolution of Morrison lakes apparently was driven by filling of the accommodation space as open-lacustrine facies aggraded and marginal-lake fades prograded into the central portions of the lake. Lacustrine deposition does not appear to have been continuous in space and time in east-central Colorado and distribution of the lakes probably represents variations in local and regional accommodation space. The lakes and ponds occupied paleotopographic lows which locally formed as a result of differential compaction of Morrison and underlying clastic sediments as well as regional subsidence associated with the Denver-Julesburg basin, which produced a slightly irregular depositional topography.

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