Abstract Previously termed concave bank-bench deposits, point-bar-tail deposits, and distal point-bar deposits, counter-point-bar deposits have received little research attention, and their stratigraphy and sedimentology is poorly understood in modern and ancient river meander belts. The stratigraphy and lateral continuity of counter-point-bar deposits were studied in the modern Peace River Delta, northeast Alberta, Canada. Morphology of counter-point-bar deposits was identified as having concave-shaped scroll-bar and vegetation patterns, and they are always located immediately downriver from sandy point bars. Stratigraphy of counter-point-bar deposits, composed of 55 to 86% silt, were studied using a vibracorer to depths of 6.5 m and channel-bottom grab samples to depths of 20 m over a river distance of 1800 m. This contrasts with 95 to 100% sand in adjacent point-bar deposits. Like point bars, counter point bars are also lateral-accretion deposits, but are composed of inclined heterolithic strata. Counter-point-bar deposits can form in short-radius meanders, as well as in wide-radius broad meanders. Counter-point-bar deposits formed where channels impinge at low angles (10 to 40°) against bedrock valley sides or other erosion resistant sediment. Within the Athabasca River Delta, a different feature called an eddy-accretion deposit, was studied and compared to counter-point-bar deposits. Eddy-accretion deposits formed as short-radius, pronounced concave scroll-bar and vegetation patterns that notably arc up-valley. In contrast to the counter-point-bar deposits analyzed, these deposits are thicker, sandier, and not as laterally extensive for a channel of a given size. Eddy-accretion deposits formed where river flow impinges more directly against (40°–140°) a valley side or channel margin composed of bedrock or resistant sediment such as an oxbow-lake fill, abandoned-channel fill or, as in this case, a Holocene prodelta deposit. The eddy-accretion deposit studied is up to 500 m wide and 24 m thick, although probably closer to 34 m thick during near-bankfull discharge. It is composed of up to 80% sand.

Abstract Deposition of the Sauk megasequence of northwestern Canada, from Peace River in British Columbia (north of lat. 56°N) to the Beaufort Sea in the Northwest Territories and Yukon Territory (lat. ∼70°N), occurred along a complex segment of the rifted western margin of Laurentia. This synthesis describes the stratigraphy, depositional history, tectonic setting, and resource potential associated with the development of the Cambrian-Ordovician great American carbonate bank. There have been few detailed studies of the Sauk megasequence in these remote areas of Canada, and much information on platform and equivalent facies remains at a reconnaissance scale. Platform rocks occur as relatively flat-lying strata of the Interior Platform that thin eastward onto the Canadian shield and correlate westward to strata within the deformed belt of the Canadian Cordillera. It is generally accepted that the Sauk megasequence was deposited along the subsiding rifted margin of Laurentia because facies pass from shallow-marine settings to deeper water slope and basinal settings from east to west. Abrupt facies changes, thickness changes, and intercalated volcanic rocks in most successions suggest a setting more complex than a simple passive margin during the development of the Cambrian-Ordovician great American carbonate bank. During Sauk megasequence deposition, positive and negative tectonic features controlled deposition along the continental margin and are represented by distinct present-day physiographic regions. The northern Rocky Mountains region lies between 56 and 60°N and is included in this summary because its geologic history is more similar to regions to the north, instead of to the southern Cordillera. North of 60°N latitude, five main regions span the vast northern Canadian mainland sedimentary basin: Beaufort-Mackenzie, Interior Plains, Mackenzie Arc, Northern Yukon, and Selwyn-Cordillera. The lower Paleozoic succession is poorly known in the “Beaufort-Mackenzie Basin,” a term that typically refers only to uppermost Cretaceous to Holocene Mackenzie Delta sediments. The northern Rocky Mountains region lies north of Peace-Athabasca arch. The arch was a positive area and forms the southern limit to the Macdonald platform and the Kechika trough, a graben system that flanked the platform to the west. The Kechika trough accumulated mainly shallow-water siliciclastic sediments until the Late Cambrian, and Sauk I and II super-sequences are characterized by complex facies changes. Carbonate deposition was prominent in the Middle Cambrian when bioherms developed on tilted fault blocks in the western part of the Kechika trough. The uppermost Cambrian–Ordovician Sauk III stratigraphy records deposition during postrift passive-margin thermal subsidence; however, during the middle Sauk III supersequence, a distinct east-to-west platform-to-basin setting evolved in response to renewed extension along the margin. The Interior Plains region lies mainly in the Northwest Territories and contains a subsurface record of the Mackenzie platform. In the Cambrian, the area was an epicontinental marine basin consisting of several depocenters that opened to the south and were segmented by positive high elements such as the Mackenzie arch. Lower and Middle Cambrian units are siliciclastic dominated and discontinuous because of the extent of paleogeographic highs. Extensive Sauk III platform carbonates of the Mackenzie platform were deposited beginning in the latest Cambrian. The Mackenzie Arc region is the eastern part of the foreland belt in Yukon and the Northwest Territories. It offers a more continuous stratigraphic record compared with the Interior Plains, but abrupt facies changes from north to south suggest phases of extension. The region contains deformed strata of the Mackenzie platform, the Mackenzie arch, and the eastern part of Selwyn Basin, which extended into the platform as Misty Creek embayment. The Sauk I supersequence is siliciclastic dominated, and carbonates are present only in distal slope settings. Sauk II and III supersequences are carbonate dominated, representing expansion of the Macdonald platform across the Mackenzie arch. Prominent slope facies in the Sauk II and III supersequences suggest an abrupt platform-to-basin setting that also accumulated submarine volcanics during phases of extension. The Selwyn-Cordilleran region extends from southwestern Yukon and northeastern British Columbia to the international border with Alaska. It is an area of deformed incompetent strata that were deposited in the Selwyn Basin, west of the Mackenzie Arc. The region contains autochthonous North American strata east of the Tintina Fault, and parautochthonous and al-lochthonous terranes west of the fault. One parautochthonous terrane thought to be a displaced slice of the North American margin is the Cassiar terrane or Cassiar platform. The Sauk mega-sequence of Selwyn Basin is generally similar to that of the Mackenzie Arc region, although the Sauk I and II supersequences contain additional units of distal basinal siliciclastic facies. The northern Yukon region contains several mountain belts and intermontane basins or lowlands that contain a succession with overall similarities to the North American Sauk mega-sequence, although the record is poorly understood in this remote area. The main tectonic element is a broad area of platform carbonates, the Yukon stable block, which is bordered to the east by the Richardson trough and to the south by the Selwyn Basin. The lower Sauk I supersequence is absent because of uplift of the Ogilvie arch. Block faulting influenced a mostly siliciclastic-dominated Sauk II supersequence, although deposition of carbonate resumed during deposition of the upper Sauk II supersequence and persisted throughout the Sauk III supersequence deposition. Sauk megasequence carbonates that may have been part of the great American carbonate bank underwent abrupt lateral facies changes indicative of syndepositional extension during the Early to Middle Cambrian, and again in the Middle Ordovician. These tectonic complexities, coupled with the lack of detailed studies, preclude a comprehensive sequence-stratigraphic analysis and determination of relative or eustatic sea level changes. The base of the Sauk megasequence is a prominent sub-Cambrian unconformity in most regions; however, eastward (toward the craton), this surface represents a merger of several unconformities, and westward (basinward), various Cambrian units overlie Proterozoic rocks. The base of the Tippecanoe megasequence, although muted and less clear in thicker shelf and offshelf successions, lies in the Middle or Upper Ordovician. The Sauk I supersequence is dominated by shallow-water siliciclastic deposits, many of which are localized in extent, with rare carbonate in distal settings such as the Selwyn Basin. Carbonate units of the middle to upper Sauk I supersequence were most likely deposited basinward of a carbonate platform. The best record of Sauk II carbonate deposition occurs along the margins of the Selwyn Basin or is represented bynarrow tracts associated with block faulting. The basal parts of the Sauk III supersequence are poorly known, but significant Upper Cambrian terrigenous siliciclastic rocks in some regions suggest a phase of uplift. The Sauk III supersequence is otherwise characterized by widespread deposition of carbonate platform and equivalent slope and basin deposits at the edge of platforms during widespread transgression and continued margin subsidence. The vast area of northwestern Canada has both hydrocarbon and mineral resource potential in rocks of the Sauk megasequence, although exploration is limited by remoteness and lack of infrastructure. Early Paleozoic intracratonic and platform-to-basin settings of the margin accumulated both potential source and reservoir rocks for fluid hydrocarbons. The Colville Hills area of the Interior Plains region has the most significant discovery of gas and some condensate within Cambrian strata. This basal Cambrian siliciclastic play potentially exists across the Interior Plains. Porous reservoir facies of Cambrian–Ordovician platform carbonates constitute another potential play, and abundant source rocks are present within equivalent basinal facies. Although mountainous regions have little hydrocarbon potential because of their geologic history, some Cambrian–Ordovician rocks host mineralized zones, particularly in the Selwyn-Cordillera region such as the metallogenic belt of Zn-Pb-Ag sedimentary exhalative deposits of the Anvil district along the western margin of the Selwyn Basin.