Enigmatic Beds of the Deep-Marine: A Depositional Continuum to Explain the Origin and Spatial Variability of Matrix-Rich Sandstones, Neoproterozoic Windermere Supergroup, Western Canada

Abstract

Although first described in the 1960's, it is only recently that matrix-rich sandstones have been widely recognized in the ancient deep-marine sedimentary record. In spite of this, their depositional origin and paleogeographic significance remain a source of ongoing debate which, not insignificantly, is because details of their lateral lithological make up are poorly known. This study describes two well-exposed outcrops of matrix-rich sandstone that can be traced continuously for 350 m and 650 m laterally in slope and proximal basin floor deposits, respectively. Significantly, lithological trends are similar in both sections. In general, the trend comprises matrix-poor (<20% matrix-content) or intermediate (20-50% matrix-content), planar- to scour-based, coarse-tail graded, medium- to coarse-grained sandstones that over ∼150-300 m transition gradually into bipartite beds made up of a lower sand-rich part overlain sharply by a mud-rich part. Over the transition, grain size and bed thickness decrease, but mud content in the upper part of the bed increases to >70%. The basal part remains ∼45% matrix. Further laterally (∼50-150 m), the upper part thickens and becomes less matrix-rich (∼50%), while the basal part progressively thins and then pinches out. Strata now consist of planar-based, massive or coarse-tail graded, fine- to medium-grained sandy claystone with ∼50% matrix. Still further laterally (∼100-200 m), matrix-content increases to ∼70% and then rapidly decreases thereafter. Additionally, strata fine (fine sand) and thin (2-3 cm) as they become thin-bedded Tcd turbidites. The observed lateral facies trend reflects an evolution of flow structure that is interpreted to be controlled primarily by particle deposition, namely sand. Near the flow axis, rapid suspension fallout of mostly sand produced thicker, coarse-tail graded beds. With diminishing sand content, the flow deposited thinner, more mud-rich beds. Fines became preferentially segregated into the top of the flow, forming an upper matrix-rich part that thickened laterally. Concomitantly, the more sand-rich basal part of the flow thinned and eventually pinched out as the flux of settling sand became negligible and the upper, matrix-rich part became the entire flow. Eventually, cohesive forces caused the flow to freeze and the dilute, turbulent tail of the flow to extend beyond the terminus of the matrix-rich strata and deposit thin-bedded turbidites.

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