Alternating Currents: When Braided Channels Begin to Meander

Abstract

Numerous papers have been published on the characteristics of braided and meandering channels. Sections exposed in Dinosaur Provincial Park and at Sandy Point, both provincial parks within Alberta, show the transition from the braided channels of the underlying Oldman Formation into the meandering channels of the overlying Dinosaur Park Formation, both Campanian in age. Detailed fieldwork at those localities, comprising numerous logged sections, have been used to build up a three dimensional picture of what happens at this transition. Several striking geological features can be directly tied to events across this boundary. The succession begins with the Comrey Sandstone, made up of several laterally extensive braided river sand bodies. Overlying this is a succession of floodplain mudstone beds, capped by a striking orange sandstone with common unionid fugichnia and extraformational clasts, in contrast to the rest of the Oldman Formation. This is overlain by further orange coloured, dipping siltstones and mudstones interpreted as lateral accretion surfaces (LAS). The contact to the overlying Dinosaur Park Formation is marked by a change to bentonitic flood plain sediments, common palaeosols and cream coloured fine grained sandstone beds representing meandering channels. This contact lies a further metre above the orange LAS, showing that the transition from braided to meandering deposits is initiated some way below the Formation boundary. Lateral changes in the character of this transition will be discussed, as well as the orientation of the orange LAS (point bar) deposits. A model will be presented that explains all of the sedimentological data over this interval. Additional analytical work has been undertaken on the modern day Bow and Missouri Rivers. These are considered representative of rivers in temperate North America. The Bow River (and downstream the South Saskatchewan River) is considered as a type example of a braided river, yet both rivers are clearly hybrid, with characteristics of both meandering and braided rivers, often within the same reach. In the Calgary area, the downtown area and several suburbs adjacent to the river inhabit point bars, while the river exhibits many braid bars as it flows through the city. The Missouri River, examined in the area around Great Falls, Montana, is similarly a “hybrid” river. The implications of these observations for the transition described above will also be discussed.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016