Paleovalley systems in the Upper Cretaceous Dunvegan Formation, Alberta and British Columbia

摘要

The Cenomanian Dunvegan Formation is divided into ten regressive-transgressive allomembers designated J-A in ascending order, and represents a large delta complex deposited over about 2 m.y. Valley systems were mapped on the upper surfaces of allomembers H to E, using 4800 well logs and 40 outcrop sections distributed over 50,000 km~2. Valleys trend broadly NW to SE across a delta plain that expanded in width from 150 to over 300 km during progradation of the deltas that comprise allomembers H to E. The longest valley can be traced for up to 330 km. A few valleys can be traced seaward into low-sinuosity distributaries feeding delta lobes. Most other valleys disappear within 20-30 km of the low-stand delta front, at a point interpreted to separate the falling stage from the lowstand systems tract. Most valley systems have a dendritic pattern, although an apparently anastomosed pattern is observed in some areas. Individual valley reaches have a strong NW-SE and NE-SW preferred orientation, with abrupt, approximately right-angle bends and junctions. These valley trends mimic those of faults in the underlying Carboniferous rocks. Valley depths range between 15 and 40 m, and average depths range from 19.1 to 23.8 m, with an overall average of 21.3 m. There is no systematic variation in depth along the length of individual valleys. Valleys are typically 1 to 2 km wide, but can expand to a maximum of 10 km, sometimes, but not always in the vicinity of confluences. Valley-fills are dominated by fine- to medium-grained sandstone, up to 97% in some wells. Sandstone probably represents multi-storey point bars. Mudstone tends to form a greater part of the fill (up to 50%), in more seaward reaches. Muddy deposits are generally concentrated in the upper 5-15 m of valley-fills, where muddier-upward units might represent heterolithic point-bars and/or tidal flats whereas sandier-upward units could represent bay-head deltas. Valley incision is considered to be most probably attributable to eustasy coupled with climatically driven changes in the ratio of sediment load to discharge. Neither tectonic tilting, nor changes in the rate of subcrustal loading appear to provide plausible explanations for incision and filling.