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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Geosciences

Year Degree Awarded

Spring 2014

First Advisor

R. Mark Leckie

Subject Categories

Geology | Paleontology | Sedimentology | Stratigraphy

Abstract

The Cenomanian/Turonian (C/T) boundary marine strata of the Late Cretaceous Western Interior Sea (WIS) exhibit a positive carbon isotopic excursion in the bulk-carbonate and organic carbon. This marks Oceanic Anoxic Event 2 (OAE 2), which spans the uppermost part of the Hartland Shale and one-third of the overlying Bridge Creek Limestone members of the Greenhorn Formation and their equivalents. The interval is characterized by alternating beds of light-colored limestone and dark-colored marlstone and calcareous shale. These lithologic couplets have been related to Milankovitch orbital cyclicity. Foraminiferal assemblages from three selected sites, including the C/T boundary Global Boundary Stratotype Section and Point (GSSP) at Rock Canyon anticline near Pueblo in central Colorado, the Cuba section in north-central Kansas, and a section near Sioux City in northwest Iowa indicate major perturbations in paleoceanography conditions associated with eustatic sea level rise and global warming. Despite the global nature of the OAE 2, the strata in the WIS indicate a local geological and oceanographic imprint. The north-south configuration of the WIS and the influence of southern Tethyan and northern Boreal water masses on the sedimentological, geochemical, and paleontological character of the seaway created a unique geological record. Foraminiferal assemblages of the Bridge Creek Limestone in the basin center record subtle cyclical alternations between limestones and adjacent marlstones or calcareous shales. The similarity of the structure of foraminiferal assemblages and their response to climatic perturbations associated with OAE 2 in the central and eastern parts of the seaway suggest that both sides were under the influence of the same, warm, southern watermass.

The organic-rich strata associated with OAE 2 in the WIS have been attributed to either enhanced surface water productivity or oxygen depletion of bottom waters as a result of fresh water runoff and water column stratification. In addition, different oceanographic scenarios have been proposed to explain the stratal and faunal variations of the Bridge Creek Limestone. However, each of these scenarios has its complexity. This study proposes an oceanographic model that invokes a cyclonic gyre operating in two different modes (strong and weak) to explain the lateral and temporal lithofacies and biofacies variations. The weakening and strengthening of the gyre circulation and changes in nutrient availability in the surface water resulted in deposition of organic-rich marlstone and limestone beds, respectively. Sea level rise during late Cenomanian and early Turonian time, and the increasing size of the WIS reduced the effect of water column mixing in the basin center increasing the preservation potential of organic matter. The correlation of the sequence of events associated with the OAE 2 in the WIS to equivalent strata in Eastbourne, UK and Ocean Drilling Program Site 1260 in the Demerara Rise Basin of the tropical Atlantic reveals the global nature of these events and importance of the WIS as an important north-south paleoceanographic corridor. During the C/T boundary interval, the WIS may have transported heat to Boreal basins as a component of Northern Hemisphere surface ocean circulation, and it may have exported intermediate waters to the proto-Gulf of Mexico and western Tethys.

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