ORIGIN AND SIGNIFICANCE OF MUD-FILLED INCISED VALLEYS (UPPER CRETACEOUS) IN SOUTHERN ALBERTA, CANADA

Seven mud-filled incised valleys (MFIVs) in the paralic facies of the Dinosaur Park and Horseshoe Canyon formations (Upper Cretaceous) of so uthern Alberta were studied to better understand their morphology, geo metry and depositional histories in an estuarine context. Two preserva tional geometries occur: simple, U-shaped forms; and internally comple x forms. Both types of MFIV record deposition in the central zone of l ow energy (turbidity) in an estuarine setting. Simple, U-shaped MFIVs hare sharp basal erosional surfaces and consist of mudstone-dominated heterolithic fills of channel-wide, concave-up laminae. Associated fos sil assemblages are marine to brackish. Each simple MFIV records a cut -and-fill history associated with a cycle of relative sea-level drop a nd rise. Low-energy depositional settings, loss of channel form during infilling, and associated shoreface deposits, as well as the absence of clear tidal indicators suggest a coastal plain estuarine setting, a long a wave-dominated, barred coastline. Complex MFIVs are rarer, and consist of imbricated, wedge-shaped sets of inclined-to-horizontal het erolithic strata. Tidal deposits and/or nonmarine-to-marine macrofossi ls occur locally. Complex MFIVs were infilled in meandering reaches of the central zone of low energy in tide-dominated estuaries. Their rar ity compared to simple MFIVs and their freshwater palaeontological con tent suggest that they were contiguous landward with extensive fluvial channels. A complex MFIV near Onefour comprises three in-channel depo sitional cycles. Each cycle consists of an erosional surface overlain by lateral accretion bedding and a conformable transition to verticall y aggraded strata. Each cycle reflects a cut-and-fill event under the control of changes in relative sea-level that culminated in overbank f looding. All MFIVs formed in low-gradient settings (less than or equal to 0 . 03%) where estuarine zones were stretched out over many tens o f kilometres. Tide-dominated estuaries apparently exhibited simple, st raight-to-meandering upstream transitions and extensive landward penet ration (greater than or equal to 200 km) of tidal backwater effects. F ew modern estuaries serve as adequate modern analogues to these ancien t, tide-dominated estuaries. Radiometric data indicate that MFIV cut-a nd-fill cycles were 100 000-400 000 years in maximum duration and thus , equivalent to 4th order sea-level cycles. However, negative evidence tentatively suggests that these cycles took place over time intervals 1-2 orders of magnitude smaller (5th order or higher sea-level cycles ).