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Construction of Tectonic Subsidence Curves for The Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for subsidence mechanisms, age of breakup, and crustal thinning GERARD C. BOND Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 MICHELLE A. KOMINZ ABSTRACT interpreted by Stewart (1971, 1972) and by sections in the thrust and fold belt of the south- Burchfiel and Davis (1972) as evidence that a ern Canadian Rockies, where the strata are ex- A quantitative procedure has been developed passive continental margin was initiated at some ceptionally well exposed and good stratigraphic for calculating tectonic subsidence in fully lithi- time in the late Precambrian along the western and structural controls have been established. fied strata and has been applied to stratigraphic edge of the North American craton. This inter- The results of the analysis are compared with sections in the early Paleozoic miogeocline of pretation was based on a comparison of the thermal-mechanical models of passive margins the southern Canadian Rocky Mountains. The geology of the miogeocline with the generalized to identify the mechanisms that controlled the results indicate that tectonic subsidence along and largely descriptive models of passive mar- subsidence and to obtain a new estimate for the inner edge of the miogeocline was controlled gins that were available at the time. the age of continental breakup and initiation of mainly by thermal contraction of heated litho- In recent years, new quantitative methods and the passive margin in the southern Canadian sphere. Comparison of a palinspastically restored geophysical models have been developed for Rockies cross section of the inner part of the miogeocline analyzing the evolution of modern passive mar- with a cross section constructed from a two- gins (Keen, 1982; Watts, 1981). In this paper, PROCEDURE FOR CALCULATING dimensional stretching model suggests that thin- the evolution of a part of the Cordilleran miogeo- TECTONIC SUBSIDENCE IN THE ned continental crust was present beneath the cline in the southern Canadian Rocky Moun- MIOGEOCLINAL STRATA inner miogeocline. These results support the tains is re-examined using a quantitative method passive-margin model that has been proposed for analyzing subsidence that has been applied The procedure we use to calculate tectonic for the miogeocline. The extensive transgression recently to modern passive margins. This meth- subsidence in the miogeoclinal strata is a modifi- onto the craton east of the miogeocline in Cam- od involves the construction of tectonic sub- cation of the backstripping method that was brian time, however, cannot be explained by sidence curves from columnar stratigraphic originally developed by Sleep (1971) and later subsidence processes operating within a passive sections (Sleep, 1971; Watts and Ryan, 1976). applied to the northwestern Atlantic margin by margin, and the transgression could be evidence Tectonic subsidence, which is subsidence caused Watts and Ryan (1976) (see Steckler and Watts, for a eustatic rise of sea level. solely by a tectonic or driving mechanism, is cal- 1978; and Sclater and Christie, 1980, for de- The form of the tectonic subsidence curves culated by quantitatively removing the subsi- tailed discussion of the method). The procedure, strongly implies that cooling of the heated litho- dence produced by nontectonic processes such shown diagrammatically in Figure 1, begins by sphere, which was initiated at the time of break- as sediment loading, sediment compaction, and restoring the lowest unit in a stratigraphic sec- up, could not have begun earlier than the latest water depth changes. The advantage of this tion, usually a formation or member, to its initial Precambrian or earliest Cambrian (555 Ma to procedure is that a graph of tectonic subsidence thickness and bulk density (unit la in Fig. 1) 600 Ma). Ages of 800 Ma to 900 Ma that have versus time, the tectonic subsidence curve, can and placing its top at a depth below sea level been assumed previously for rifting in the mio- be compared with subsidence curves calculated corresponding to the average depth of water in geocline are too old to have led directly to con- from various geophysical models of passive which the unit was deposited (Wd|). The iso- tinental breakup. Scattered occurrences of mafic margins. The comparison of such curves has static subsidence of the basement caused by the volcanics interlayered with arkosic sediments been found to be especially useful as a means of weight of the sediment in the unit is then re- have been reported in the latest Precambrian identifying the thermal component of subsidence moved, and the depth to the surface on which to earliest Cambrian Hamill Group exposed in and its timing in the postrift cooling stages of the unit was deposited (Yi) is recalculated with the middle to outer part of the miogeocline. modern passive margins (Sleep, 1971; Keen, only the weight of water as the basement load- These deposits may record the phase of rifting 1979; Watts and Steckler, 1979). ing factor. Next, the second unit in the section is that immediately preceded formation of the This paper begins with the development of a restored to its initial thickness and bulk density proto-Pacific margin in the southern Canadian new method that has been briefly described pre- (unit 2a) and is positioned with its top at the Rockies. viously (Bond and Kominz, 1981; Kominz and average depth of deposition of the unit (Wd2) Bond, 1982) for recovering the tectonic compo- and its base in contact with the first unit. The INTRODUCTION nent of subsidence from fully lithified and litho- thickness and bulk density of the first unit are logically diverse strata of the type that are adjusted (unit lb) in accordance with the depth The presence of an early Paleozoic miogeo- typically present in the Cordilleran miogeocline. of burial beneath the second unit. The isostatic cline in the North American Cordillera was We apply this method to selected stratigraphic subsidence due to the weight of sediment in both Geological Society of America Bulletin, v. 95, p. 155-173, 11 figs., 1 table, February 1984. 155 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/2/155/3434462/i0016-7606-95-2-155.pdf by guest on 25 September 2021 156 BOND AND KOMINZ Unit 2 Restored to Fully Unit 1 Sediment Initial Thickness Sediment in Lithified Restored to in Unit 1 and Density; Units 1 and 2 Section Initial Isostatically Unit la Adjusted Isostatically TIME- Thickness Removed for Depth of Burial Removed /VV\ and Density Beneath Unit 2a. '5' ' SEA ^ Y0 LEVEL WATER 4. • ESSijWd, T, DEPTH 3. Ma' 2a- • / \ /\ V \ ' 1b. •BASEMENT \ Figure 1. Steps in the procedure for calculating tectonic sub- Vv' sidence in the fully lithified strata of the miogeocline. Correction / / / V v for eustatic changes in sea level not shown. See text for ^ = Age in Ma for ' V / Stratigraphie Boundary discussion. the first and second units is removed as before, loads to tectonic subsidence, and Wd = average Pray, 1970; Friedman, 1975). In other litholo- and the depth to the surface on which the first depth of water in which unit was deposited. gies in the miogeocline, however, compaction unit was deposited (Y2) is again recalculated This equation is not directly applicable to the appears to have been important, especially in with only the weight of water as the loading miogeoclinal sequences, however, because of the shales and fine-grained limestones containing factor. These steps are repeated for successively way in which S*, the decompacted sediment fluid escape structures and flattened burrows. To higher units until they are completed for the thickness, has been calculated. For modern ba- calculate S* for the miogeoclinal strata, there- highest unit in the section. To complete the sins, calculation of S* has been simplified by fore, the sedimentary layers must be delithified; procedure, the water depths should be corrected assuming that thicknesses and porosities of the that is, corrections must be made for both com- for any eustatic changes in sea level, if known, sedimentary layers decrease during burial solely paction and cementation during burial, and using the method described by Steckler and by compaction resulting from mechanical pro- these corrections must be developed without Watts (1978) (this step not shown in Fig. 1). cesses and/or pressure solution with precipita- direct porosity measurements as a guide. The tectonic subsidence curve for the section is tion of all dissolved material in pore space (Van given by plotting the values of Y as a function of Hinte, 1978; Perrier and Quiblier, 1974; Steckler A Procedure for Delithifying the radiometric ages of the stratigraphic units (Fig. and Watts, 1978; Sclater and Christie, 1980). Miogeoclinal Strata 1). If the base of unit 1 was not deposited close The change in thickness and density of a layer to sea level, the first point on the tectonic during burial, therefore, is assumed to be directly The quantitative relations between depth of subsidence curve (Y0) must be plotted at the proportional to the decrease in porosity in that burial, compaction, and cementation are poorly water depth for the base of that unit. layer with depth. The decrease in porosity with understood, and it is not possible to specify a depth is calculated using well-log data from drill precise empirical or theoretical delithification Calculation of Tectonic Subsidence holes in the margin. The change in porosity with factor for each lithology in the miogeocline. depth cannot be determined in this manner for Quantitative delithification factors cannot be ob- The calculation of tectonic subsidence in a the sedimentary rocks in the miogeocline be- tained directly from petrologic studies of the unit of a stratigraphic section is given by the cause their long exposure to diagenesis has miogeoclinal sediments, either, because of the general equation that has been developed for reduced the porosities to essentially zero.
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