麻豆传媒

听

The Upper Ordovician (Ashgillian) to Middle Silurian (Wenlockian) graptolitic carbonates and cherts of the Cape Phillips Formation accumulated in the deep water basinal environment of the Hazen Trough. The coeval platform carbonates of the Allen Bay Formation to the east in a shelf environment. Nineteen stratigraphic sections were measured in order to obtain data on the lithological variation within this facies transitions, specifically within the Cape Phillips Formation. Integrated petrological and x-ray diffraction analyses of the samples collected indicates the presence of seven distinct lithofacies, six in the Cape Phillips and one in the Allen Bay Formation. These lithofacies are;

1. laminated limestone
2. calcareous mudstone
3. laminated argillaceous limestone
4. massive limestone and rudaceous graded limestone
5. chert
6. laminated dolomite
7. massive dolomite

The distribution of these lithofacies is strongly related to the paleogeography and water depth. The shallow water carbonate buildup of the Allen Bay Formation is composed of massive dolomite. The slope carbonates of the Cape Phillips Formation consist of laminated dolomite and laminated limestone with minor massive limestone and rudaceous graded limestone. The basin is dominated by calcareous mudstones and displays a significant development of chert.

To explain the distribution and origin of the lithofacies, a model is proposed whereby the carbonate grains and mud were produced on a shelf platform and carried basinward by wave - and tide - generated currents. Massive limestone and rudaceous graded limestone are the proximal deposits of gravity flows. Laminated limestone and laminated argillaceous limestone are the distal deposits of gravity flows. Calcareous mudstone and radiolarian chert were deposited from hemipelagic and pelagic suspensions and turbid layers. Replacement chert resulted from the silicification of carbonate. Dolomitization of the platform carbonate buildup and the adjacent upper slope succession created the massive dolomite and laminated dolomite lithofacies respectively. Dolomitization may have occurred in a schizohaline environment.

The dominance of calcareous mudstone in the upper Cape Phillips compared to laminated limestone, which is dominant in the lower Cape Phillips, suggests a deepening of the basin and a general transgression during the accumulation of the Cape Phillips Formation. It appears to have been deposited in relatively deep water, at least below wave base and generally above the carbonate compensation depth, between 100m and 5400m deep.

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Pages: 117
Supervisor:听 Martin Gibling