Sedimentary environments and structure of the Cretaceous rocks of Saturna and Tumbo Islands, British Columbia Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r494vp751

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  • Seven formations of the Upper Cretaceous Nanaimo Group sedimentary rocks are exposed on Saturna and Tumbo Islands. These formations are, from oldest to youngest, the Extension-Protection, Cedar District, DeCourcy, Northumberland, Geoffrey, Spray, and Gabriola. The rocks represent four successive cycles of deltaic sedimentation, the oldest and the youngest of which are incompletely exposed in the thesis area. A composite section of maximum thickneis for the formations exposed on Saturna and Tumbo Islands is 9, 776 feet; however, the formations vary significantly in thickness along strike. The Extension-Protection Formation, consisting of conglomerates and lithic wackes, represents the upper part of a delta complex. The lower part of the cycle is not exposed in the thesis area. The conglomerates are interpreted as having been deposited in the bed load and point bar subenvironments of highly competent streams. The sandstones overlying the conglomerates are interpreted as marine topset sands. The Cedar District Formation, the lower part of the Cedar District-DeCourcy deltaic cycle, overlies the Extension-Protection Formation with an angular discordance of about 50. The Cedar District Formation consists of repetitively interbedded and normally graded sandstones, siltstones, and mudstones, which were probably deposited as turbidites. The Cedar District strata are interpreted as delta-slope deposits. Conformably overlying and intertonguing with the Cedar District Formation are the channel-mouth bar arkosic and lithic wackes of the DeCourcy Formation. Distributary channel conglomerates overlie the marine sandstones which are in turn overlain by marine channelmouth bar sandstones. Pa leocurrent data indicate that the Cedar District-DeCourcy deltaic complex prograded in an east-southeasterly direction. The Northumberland Formation, the lower part of the Northumberland-Geoffrey deltaic cycle, conconformably overlies the DeCourcy Formation. Most of the Northumberland Formation was deposited as prodelta mudstones. However, the lower and upper parts of the formation were deposited in delta-slope environments, indicating a transition from and to the marine sandstones of the DeCourcy and Geoffrey Formations, respectively. The Geoffrey Formation intertongues with the Northumberland Formation and consists almost entirely of channel-mouth bar sandstones with local distributary channel conglomerates. The source of the sediments for the Northumberland-Geoffrey deltaic cycle was to the west or northwest. Paleocurrent data indicate that the sandstones of the Geoffrey Formation were distributed in a north to south direction around the mouths of distributary channels by longshore currents. Overlying and intertonguing with the Geoffrey Formation is the predominantly mudstone Spray Formation, most of which is not exposed in the thesis area. The Spray Formation was deposited as turbidites and/or seasonal flood deposits in a delta-slope environment. Overlying and intertonguing with the Spray Formation is the Gabriola Formation, the youngest formation exposed in the thesis area. The Gabriola Formation consists of bed load and point bar fluvial conglomerates, marine channel-mouth bar arkosic and lithic arenites, and topset conglomerates. Lithologies of the sandstones, pebble counts of the conglomerates, and sedimentary structures indicate a paleocurrent dispersal direction to the east or northeast for the Spray-Gabriola deltaic cycle. At least two major episodes of structural deformation have been recorded on Saturna Island. The first episode of structural deformations resulted in a series of north-south trending faults. The second episode of structural deformation resulted in a series of east-west trending faults and related folds (the Kulleet Syncline).
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  • File scanned at 300 ppi (Monochrome, 24-bit Color) using Capture Perfect 3.0.82 on a Canon DR-9080C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR. Plate: Master map file scanned at 600 dpi, 24-bit color on a Paradigm ImagePRO GxT 42 HD (OEM version of ColortracSmartLF Bx 42). Image manipulated by SmartLF1.3.05.
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