Architecture and depositional history of the Lower cloridorme formation, Gaspé Peninsula, Québec, Canada

Awadallah, Sherif Abdel Monem (2002) Architecture and depositional history of the Lower cloridorme formation, Gaspé Peninsula, Québec, Canada. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The lower Cloridorme Formation (Middle Ordovician) is superbly exposed along the southern shore of the St. Lawrence River. It is dominated by shale and is characterised by four turbidite architectural elements: megaturbidites, lobes, sandstone sheets and siltstone lenses. The lower Cloridorme Formation was deposited in a deep-marine setting akin to modem basin plains and distal submarine fan systems. The deep-marine setting was a foreland basin that developed in front of advancing thrust sheets during the Taconic Orogeny. The depositional environment evolved from a basin-plain setting to a submarine- fan setting, probably due to progradation of the turbidite system from the east. -- The detailed correlation of more than seventy basin-wide megaturbidites and nine K-bentonite horizons permits the division of the lower part of the Cloridorme Formation into seven time-equivalent intervals, called "time-slices", that vary spatially and temporally in their thickness, constituent facies and architectural elements. The main contribution of this thesis is the precise mapping and correlation of architectural elements in the lower Cloridorme Formation using the key beds and ash horizons, leading a better understanding of the depositional history of the lower Cloridorme Formation. -- Megaturbidites are the most common architectural element. Megaturbidites were deposited from large flows initiated by major sediment failures that evolved into turbidity currents. Some of these megaturbidites were deflected and reflected against the basin margins and bathymetric highs. Megaturbidite geometries were controlled by bottom topography and range from tabular to wedge shaped to gently lensing. The megaturbidites were deposited in basin-plain and the lower-fan environments. Megaturbidites are bounded by single event boundaries. -- Lobe elements consist of structureless, mostly amalgamated sandstone beds that form 10-20 m-thick packets surrounded by shale. The sandstone beds were deposited from concentrated flows that did not travel long distances and were strongly controlled by bottom topography. The lobes formed mounds that controlled the flow of subsequent turbidity currents. The lobes have a tabular or a gently lensing geometry over distances of 1-3 km, but taper over longer distances. These lobes were deposited in a lower fan environment during periods of increased supply of coarse sediment to the area. The packets of beds that represent these lobes consist mostly of structureless sand in amalgamated units. These packets are bounded by erosional surfaces in most cases. -- Sandstone sheets are not common. They were deposited on a basin plain as a variety of sandstone facies that range from structureless to well laminated. The sheets also contain laminated siltstone beds. The sheets become more muddy in a downcurrent direction. They have a tabular geometry and probably developed during periods of increased sand and silt supply reaching the area. Many of the beds in these sheets do not form amalgamated units. These sheets are bounded by erosional or non-erosional surfaces. -- Siltstone lenses consist of laminated siltstone beds. They become more muddy and less distinct in a downcurrent direction. Over distances of 1-3 km, these deposits are tabular or have a subdued lens shape. These lenses occur in both the basin plain and lower fan and are bounded by erosional or non-erosional surfaces. -- A suggested depositional history of the lower Cloridorme Formation is provided which shows the influence of bottom topography on the distribution of the different facies and the geometry of beds and architectural elements. Seafloor irregularities were the result of the combined effects of tectonic, depositional and compactional factors. Deposition was influenced or controlled by local (autocyclic) factors such as charmel switching and more regional (allocyclic) factors such as tectonics and sea-level changes. -- The bed-thickness distributions of the turbidite beds of the lower Cloridorme Formation vary from mainly lognormal for the sandstone beds to exponential for the siltstone beds. The observed type of distribution is in many cases the summation of a mixture of subpopulations. Some of the bed-thickness populations approximate power-law distributions with a scaling parameter ranging from 1-2, but this fit to a power-law only applies to a subset of beds that represent a small part of the population. Thinner beds that form a greater part of the population might also follow a power-law distribution but have a much smaller power-law scaling parameter than the associated thicker beds. A comprehensive understanding of the depositional conditions and the factors that control the thicknesses of beds is required before attempting to infer submarine-fan subenvironments and triggering mechanisms for turbidity currents from the bed-thickness distribution.

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