Tectonic Control of Sedimentation and the Interpretation of Sediment Alternation in the Tertiary of Prince Charles Foreland, Spitsbergen

摘要

Over 6000 feet of clastic sediments, probably of early Tertiary age, have been deposited in an actively developing graben. The sequence consists of beds mostly less than a foot thick, ranging in texture from conglomerate to shale, but dominantly of low rank graywacke. While much of the rapid vertical variation in lithology is erratic, some cyclic patterns are present. The sediments were derived from an area of low-grade metamorphic rocks only a short distance to the west and were deposited at the edge of a mountainous land which was actively rising by warping and perhaps by early movements on the faults that now separate the horst of Prince Charles Foreland from the graben of the Foreland Sound. Slump structures indicate considerable downward sliding of the sediments toward the center of the graben. Lithologic cycles in the sediments have been analyzed quantitatively in an attempt to assess their significance. Major cycles are attributed to variation in the balance between the rate of upward movement of the source area and the rate at which erosion reduced the resulting topographic relief. Minor cycles (mainly conglomerate-sandstone alternations and sandstone-shale alternations) are superimposed on the major cycles. Chiefly from an analysis of the ratios of thicknesses of beds in these cycles, it is argued that they represent short-term meteorologic variations. For each kind of rock, the thicknesses of beds have a log normal frequency distribution, and the geometric mean thicknesses increase with coarseness: shales 0.3 foot, siltstones 0.6 foot, sandstones 1.0 foot, conglomerates 1.4 feet. The sandstone thickness distribution, however, is a composite of two populations. The sandstones that are in cyclic alternation with conglomerates have a geometric mean thickness greater than that of the conglomerates; the sandstones that are in cyclic alternation with shales have a geometric mean thickness less than that of the shales. These data are compared with information on bedding thickness from other areas, and the interpretation of such data is discussed. The tectonic setting is taphrogeosynclinal; the rocks are dominantly low rank graywackes. The association of various kinds of sedimentary rocks with this tectonic setting is discussed. If general uplift occurs at the same time as the differential movement that forms the taphrogeosyncline, non-marine deposits will probably develop. Arkoses will be abundant if the source area consists of high-grade metamorphic and granitic rocks. Arkoses and non-marine deposits have developed in the past in a number of situations where block-faulting accompanied the uplift and deep erosion of the central part of an orogenic belt after an episode of orogenesis. In Prince Charles Foreland, however, the horst rose and the graben sank about a mean horizon of roughly constant elevation, and the "accidental" location on the western flank of the much older Caledonian fold belt of Spitsbergen resulted in a source area of low-grade metamorphic rocks and a sedimentary pile dominated by low rank graywackes.