Wandering gravel-bed rivers and high-constructive stable channel sandy fluvial systems in the Ross River area,Yukon Territory,Canada

Mid-Cretaceous strata within the Tintina Trench, 3 km west of the community of Ross River, contain evidence of deposition in two distinct, alternating, fluvial settings. Coal-bearing, mud-dominated strata are commonly associated with high-constructive sandy channel systems, with extensive overbank, levee and splay deposits. Channels are between 3 and 30 m wide and 0.4-7 m thick. They show repetitive development of side and in-channel bar-forms, as well as up-channel widening of the rivers by selective erosion of associated overbank and levee deposits. Levees extended for several hundred metres away from the channels. In this setting low-angle inclined stratification and epsilon cross stratification may reflect lateral migration of crevasse channels or small streams. The paucity of exposure prevents recognition of the channels as products of multiple channel anastomosed systems or single channel high-constructive systems. Gravel-dominated strata, inter-bedded with, and overlying coal-bearing units, are interpreted as deposits of wandering gravel-bed rivers, with sinuosity approaching 1.4. In most exposures they appear to be dominated by massive and thin planar-bedded granule to small pebble conglomerates, which would traditionally be interpreted as sheet-flood or longitudinal bar deposits of a high-gradient braided stream or alluvial fan. Architectural analysis of exposures in an open-pit shows that the predominance of flat bedding is an artefact of the geometry of the roadside exposures. In the pit the conglomerates are dominated by large scale cross stratification on a scale of 1-5.5 m. These appear to have developed as downstream and lateral accretion elements on side-bars and on in-channel bars in water depths of 2-12 m. Stacking of strata on domed 3rd order surfaces suggests development of longitudinal in-channel bar complexes similar to those observed in parts of the modern Rhone River system. Mudstone preserved in some of the channels reflects intervals of channel abandonment or avulsion. Minimum channel width is from 70 to 450 m.

The Pliocene-Recent Euphrates river system:Sediment facies and architecture as an analogue for subsurface reservoirs

The Tigris-Euphrates is a continental-scale fluvial system,around 2800 km in length,which drains over 1 million km2 of SWAsia.The system originated in the Late Miocene and developed into the principal axial drainage system of the region,which follows broad regional structural features of the Mesopotamian Foreland Basin.Good preservation and outcropping of the Pliocene and Quaternary Euphrates deposits yield a viable local analogue for subsurface fluvial reservoirs in the region,and for other fluvial systems that have developed in foreland basin settings.This paper documents the first detailed study of the sedimentary characteristics of these Pliocene and Quaternary fluvial deposits along the middle reaches of the present-day Euphrates in Syria.The Euphrates fluvial system developed from small and probably short-lived isolated cut-and-fill channels in the Pliocene,characterised by abundant debrite and slump facies,through to a broad meandering system at present.The Quaternary deposits represent a braided to meandering system that was more energetic than that of the modern day Euphrates.The Quaternary facies include a dominance of gravels,pebbly sands and sands as channel associations,coupled with sands,muds and paleosols representing channel abandonment,overbank and crevasse-splay associations.Channel widths,where observed,range from 50 to 500 m,and minimum fill thicknesses range from 3 to 7 m.The combined channel-fill for stacked channels is up to 25 m thick.Lateral correlation of channel elements over at least 1 km of section indicates that rapid and extensive lateral migration has occurred.Crevasse splay lobes can be identified in the overbank deposits,with a width of 30e60 m and sand thickness of 0.5e1.5 m.The geometry,nature and dimensions of these architectural elements provide a useful analogue for subsurface reservoirs.The high-energy channel facies of the Quaternary system show very good reservoir attributes,with good correlation and connectivity.At the bed-scale there is significant heterogeneity of characteristics that would impact fluid-flow for hydrocarbon production from a subsurface reservoir.However,incomplete preservation of these Quaternary fluvial deposits at outcrop remains a challenge for accurately determining the scale of sedimentary features and also the size of the paleo-river.