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Fundy Basin GSC Open File 3652P.N. Moir and J. A. WadeGeological Survey of Canada (Atlantic) P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2 ![]() Cape Blomidon, elevation 215m, at the eastern end of North Mountain, over looks Minas Basin. It consists of a cap of North Mountain Basalt overlying Blomidon Formation shales and Wolfville Formation fluvial sandstones. The Triassic/Jurassic boundary occurs a few metres below the base of the basalt. INTRODUCTION This Open File is a digital release of maps prepared during the geological/geophysical study of Fundy Basin (1993-95). It consists of digital maps and figure files in various formats on one Compact Disc. The seismic tracks were derived from digitized shotpoint bases. Five seismic time and isochron maps were constructed from interpretation of the network of multifold seismic data acquired by Mobil Oil Canada, Ltd. and Chevron Standard Ltd. from 1968 to 1982. They were compiled on 1:200,000 Lambert Conformable work sheets and depth converted as described below. The interpretative maps were then digitized using the geographic
information system (GIS) software Caris
(on a Sun Solaris computer). The files were converted to UTM projection,
zone 20 and subsequently 4 structure maps and 1 bathymetry
map were converted to Autocad DXF format, and HP hpgl
plot files (200,000 and 1,000,000 scales). The hpgl files were imported
into Coreldraw ( version 7 ) to help create figures included.
METHOD
The conversion of seismic time to (1) depth to base Mesozoic and (2) top of North Mountain Basalt for Fundy Basin, used a variety of data and methods. Over most of the area, the seismic records from Fundy Basin lack a distinctive reflection from the pre-Mesozoic surface due in part to similarities in velocity between the adjacent facies. Meguma (basement) velocities have been measured from 5.3-6.5 km/s (Coflin, 1998; Keen et al., 1990) and the lower part of overlying Wolfville Formation in the Chinampas well has a measured velocity of 5.5 km/s. Similarly, measured velocity data from the Scots Bay Formation is 3.5 km/s and from the North Mountain Basalt is 4.7 km/s (Mobil Oil Canada, Ltd., 1975). The various velocity data were combined to develop a series of velocity versus depth curves for the variety of stratigraphic and thickness situations which occur throughout the basin. A base of Mesozoic pick on the seismic data was developed first for those lines approaching North Mountain, Nova Scotia, where basement could be projected into the bay using known formation thicknesses and measured dips. In several cases this projected surface was found to coincide with a zone of character-change on the seismic records between fair to weakly coherent energy (assumed Wolfville Formation sandstones and shales) and generally incoherent energy (assumed Meguma Group metasediments). This surface was tied throughout the seismic data set. Measured variations in formation dips and thicknesses from the Annapolis Valley and North Mountain, when projected toward the basin axis, agree well with the seismically derived depths from this surface. The surface of the North Mountain Basalt generates a strong reflection over most of the basin. However, where it occurs shallower than 0.7-0.8 ms, two-way-time (<~1000 m), it is typically very difficult to detect on existing seismic records. For this reason, the northern limit of the basalt is somewhat uncertain. There is a moderate to strong impedence contrast between the basalt and the underlying shales and sandstones of the Blomidon Formation making the interpretation of thickness of this unit quite straight forward. The Blomidon and Wolfville formations were penetrated by both the Chinampas and Cape Spencer offshore wells. In spite of seismic ties with the two wells, velocity contrasts between these formations are so slight that they cannot be differentiated on existing seismic data. However, an isopach map has been constructed of the interval between the base of the North Mountain Basalt and interpreted basement, i.e. the combined Wolfville and Blomidon formations. This map shows a northwest thickening wedge that culminates in the Fundy Subbasin with at least 6 km of strata, half of which is arbitrarily assigned to the Blomidon Formation. However, in Chignecto Subbasin, the interval interpreted as representing the Blomidon Formation thickens substantially into the border fault indicating a period with a higher rate of subsidence and associated basin asymmetry. In this portion of the subbasin, the Blomidon Formation is somewhat thicker than the Wolfville Formation. The Scots Bay/McCoy Brook formations are widespread in the basin, but no separation of facies is possible from the seismic data. Continuity exists, through the network of seismic profiles, from near the type section westward to the bounding faults. Reflections from within the Scots Bay Formation are weak and discontinuous but indicate general conformity with the North Mountain Basalt in the central part of the basin and minor onlap toward the southern margin. The occurrence of the formation in the erosional outlier in Chignecto Subbasin is based on correlation of seismic data from both sides of the extension of the Cobequid Fault. The northeastern limit is speculative. The seismic data indicate that more than 2500 m of post-North Mountain Basalt strata remain in the Fundy Subbasin. Erosion, of up to 2 km of the Scots Bay/McCoy Brook formations, at the seafloor can be documented using the seismic profiles. Extrapolation of beds indicates an original depositional limit along South Mountain. Five of the 8 maps in .CDR format were derived from the Caris digital files. The latter were imported into Coreldraw and generalized, through the deletion of some faults and contours, to make text figures for the Wade et al. (1996) report. Thus they are accurate, although larger scale, versions of the original maps. The North Mountain Basalt isopach was not digitized but photo-reduced to text figure size and scanned into Coreldraw. Seismic interpretation and map preparation was the responsibility of J.A. Wade. Digital file preparation, conversion and these Web pages was completed by P.N. Moir. References:
Keen, C.E., Loncarevic, B.D., Reid, I., Woodside, J., Haworth, R.T. and Williams, H. 1990. Tectonic and geophysical overview. In Geology of the continental margin of eastern Canada. Edited by M.J. Keen and G.L. Williams. Geological Survey of Canada, Geology of Canada no.2, pp. 31-85 (also Geological Society of America, The Geology of North America, I-1). Mobil Oil Canada, Ltd. 1975. Mobil-Gulf Chinampas N-37. Well History Report, Canada-Nova Scotia Offshore Petroleum Board, Halifax, Nova Scotia, 63 p. with enclosures. Wade, J.A., Brown, D.E., Traverse, A. and Fensome, R.A. 1996. The Triassic-Jurassic Fundy Basin, eastern Canada: regional setting, stratigraphy and hydrocarbon potential. Atlantic Geology, 32, pp. 189-231. |