Delta-Plain Paleodrainage Patterns Reflect Small-Scale Fault Movement and Subtle Forebulge Uplift

Abstract: The extreme sensitivity of rivers to subtle changes of slope resulting from tectonic tilting provides a basis for the interpretation of four Cretaceous paleovalley systems mapped on four successive sequence-bounding surfaces in the mid-Cenomanian Dunvegan Formation, deposited in the foredeep of the Western Canada Foreland Basin. The northern part of the study area overlies the Peace River Arch, a long-lived structural entity that is transected by faults trending dominantly NW–SE and NE–SW. The Dunvegan Fm. comprises a series of at least ten deltaic sequences labeled J–A in ascending order, and each spans ~ 100–200 ky. Valleys mapped on the top of sequences F, G, and H have many straight reaches that tend to parallel the rectilinear fault pattern in the underlying Paleozoic rocks, and many reaches are colinear with deep-seated faults. There is no evidence for deflection of rivers along the axis of the foreland basin. Isopach maps show that sequences H, G, and F do not thicken appreciably into the foredeep, suggesting that the contemporaneous rate of plate flexure was low. In-plane stress level is also inferred to have been low. Under these conditions, the forebulge was suppressed and long-lived faults in the Peace River Arch area were “unlocked”, permitting individual blocks to undergo vertical isostatic adjustment on a scale of meters. The low-gradient coastal-plain rivers were strongly influenced by this tectonic topography. In contrast, sequences E–A show marked westward thickening into a newly developed foredeep. Paleovalleys on sequence E do not have a preferred rectilinear pattern, instead being dominantly dendritic; distally, valleys are abruptly deflected to flow parallel to the basin axis. This new paleodrainage pattern is interpreted to reflect subtle uplift of a forebulge in response to renewed plate flexure, causing river deflection to the SE. The level of in-plane stress is inferred to have increased at this time, “locking” faults over the Arch and causing the land surface to deform as a regionally homogeneous plane, favoring dendritic drainage. The inferred position of the forebulge approximates the position of a long-lived Cretaceous paleodrainage divide and also coincides with a terrane suture in the Precambrian basement. The suture is inferred to have been a lithospheric weak zone, repeatedly serving to localize the forebulge. Although vertical displacement on both faults and forebulge cannot conclusively be recognized in cross sections or isopach patterns, it appears that rivers were readily diverted by these extremely subtle topographic features.

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