Base-Level Buffers and Buttresses: A Model for Upstream Versus Downstream Control on Fluvial Geometry and Architecture Within Sequences

Abstract

The effects of downstream base-level control on fluvial architecture and geometry are well explored in several broadly similar sequence-stratigraphic models. Cretaceous Dakota Group strata, U.S. Western Interior, have characteristics reflecting combined downstream and upstream base-level controls that these models cannot address. Particularly, three layers of amalgamated channel-belt sandstone within this group thicken and are continuous for distances (# 300 km) along dip that stretch the reasonable lengths for which these models are intended to apply. As well, architecture in up-dip reaches records repeated valley-scale cut-and-fill cycles. This contrasts with equivalent strata down dip which record channel-scale lateral migration with no such valley-scale cycles apparent.We here introduce the concept of ''buffers and buttresses'' to address these observations. We assume that river longitudinal profiles are each anchored down dip to some physical barrier (e.g., the sea strand, etc.) that we refer to as a ''buttress.'' Buttress shift is considered the primary downstream control on base level. Profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high-frequency variability in upstream base-level controls (e.g., discharge, etc.). All these potential profiles however are bounded above by the profile of highest possible aggradation, and below by the profile of maximum possible incision. These upper and lower profiles are ''buffers,'' and they envelop the available fluvial preservation space. Thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance. Dakota valley-scale surfaces record repeated cut-and-fill cycles driven by up-dip controls and are confined between thick stable buffers. Equivalent strata down dip record lateral reworking within a thinner channel-scale buffer zone that was positioned by downstream controls. Regression exposed slopes similar to the buffer zone, thus buffers were stable for long distances and durations. This prompted dip-extensive lateral reworking of strata into upstream valley-scale and downstream channel-scale sheets.Buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs. downstream base-level controls on geometry and architecture. The model lends general insights into dip-oriented variations in fluvial architecture, production of sheet vs. lens geometry, total preservation volumes for fluvial systems, and variations in these factors related to contrasting climatic conditions and basin physiography. The model can be amended to existing sequence stratigraphic approaches in order to capture dip-oriented variations in sequence architecture.

Towards the standardization of sequence stratigraphy

“…Away from coastal areas, sequence-bounding unconformities may also form independently of shoreline shifts, for example in relation to upstream-controlled fluvial processes (e.g., Shanley et al, 1992;Blum and Törnqvist, 2000;Catuneanu and Elango, 2001;Holbrook et al, 2006) or offshore sub-basin tectonism (e.g., Fiduk et al, 1999). Units bounded by such unconformities are "sequences" in the generic sense of unconformity-bounded units, but their internal architecture cannot be described in terms of conventional systems tracts because they lack the genetic link with coastal processes and trajectories.…”

“…by marine base-level fluctuations (Shanley and McCabe, 1991, 1993, 1994Shanley et al, 1992;Posamentier and Allen, 1999;Blum and Törnqvist, 2000;Catuneanu, 2006;Holbrook et al, 2006).…”

Responses of River Deltas to Sea-Level and Supply Forcing: Autostratigraphic View

“…This concept, originally advocated by G. K. Gilbert in the late 19th century, is often presented as the consequence of long-term equilibrium response of a river system subject to stationary sea level. Common beliefs based on equilibrium response Holbrook et al, 2006) are that (1) alluvial rivers in deltaic settings aggrade in response to sea-level rise and degrade in response to sea-level fall, (2) as long as sea level remains stationary, the rivers eventually become graded, and thus (3) grade represents the equilibrium configuration of an alluvial river under conditions of stationary sea level. Such conceptual models of graded rivers downplay the fate of the sediment bypassed through the "graded" reach, and in particular, how its sequestration in the deltaic environment affects the dynamics of the attached river system.…”

新しい平衡河川観：モデル実験が描く下流域沖積系の海水準応答

“…A/S A/S ratio concept Jervey, 1988Jervey, 1988Shanley and McCabe, 1994;Posamentier and Allen, 1999 stratigraphic architecture Wright and Marriot, 1993;Currie, 1997;Martinsen et al, 1999van Wagoner et al, 1990;Shanley and McCabe, 1994;Holbrook et al, e.g., Weller, 1960;Van Andel and Curray, 1960;Sloss, 1962;Curray, 1964;Curtis, 1970;Swift et al, 1971 A/S Exxon Group Vail et al, 1977;Mitchum et al, 1977a, b;Brown andFisher, 1977 Posamentier andNeal and Abreu, 2009 A/S e.g., Shanley and McCabe, 1994 e.g., Dalrymple et al, 1992;Ainsworth and Walker, 1994 e.g., Thomas and Anderson, 1994;Zaitlin et al, 1994;Talling, 1998 e.g., Feeley et al, 1990;Kolla and Perlmutter, 1993;Olsen, 2002 Swift andThorne, 1991;Thorne and Swift, 1991a, b Galloway, 1989a, b Helland-Hansen and Gjelberg, 1994Helland-Hansen, 1995;Helland-Hansen and Martinsen, 1996 Exxon Group A/S non-equilibrium response Muto et al, 2007 Fig. .…”