Changes in Depositional Processes--An Ingredient in a New Generation of Sequence-Stratigraphic Models

Abstract

Existing sequence-stratigraphic models for shoreline deposits commonly assume a constant process regime throughout the relative-sea-level (RSL) cycle over a wide range of timescales (third-order cycles as well as high-frequency, fourth-and fifth-order cycles), with the possible exception that tidal processes may be more important during transgressions. However, the dominant process affecting the coastal zone is a function of multiple interdependent factors and can change at any time during high-or low-frequency RSL cycles; indeed, changes are possible on timescales as short as 1,000 years. Thus, the relative intensity of wave and tidal processes may change gradually or abruptly on a regional scale because of changes in bathymetry or coastal morphology caused by rising or falling water levels, and/or changing shelf width.The specific nature of the response varies as a function of the physiographic and tectonic setting because the attenuation or amplification of wave and tidal action is strongly dependent on local and regional bathymetry and coastal morphology. Fluvial energy may also vary with sea-level change, as a result of climate change.Moreover, whereas most facies and sequence-stratigraphic reconstructions are based on river-, wave-or tide-dominated endmember environmental models, the majority of real-world environments are of mixed-energy character where these processes coexist in subequal proportions. In such mixed-energy coastal environments, changes in the relative intensity of the depositional processes on a local scale can also cause stratigraphic variations in the nature of the deposits as the environments migrate laterally.Reconstructions of regional or local process changes are complicated by the fact that changes in the grain size delivered to the coast, as a result of systematic variations in fluvial accommodation and the degree of bypass, may cause product changes without any change in the processes. Fine and very fine sand, such as is dominantly delivered to the transgressive and highstand coastlines favors the preservation of wave-generated hummocky and swaly cross stratification, whereas medium and coarse sand, as is delivered to the falling stage and early lowstand coastlines, permits the development of current-generated (including tide-driven) cross stratification. Future sequence-stratigraphic models need to incorporate and be more sensitive to such process and product changes.

Preserved Stratigraphic Architecture and Evolution of A Net-Transgressive Mixed Wave- and Tide-Influenced Coastal System: The Cliff House Sandstone, Northwestern New Mexico, U.S.A.

“…Facies partitioning into different components of a stratigraphic cycle may reflect temporal variations in the depositional-process regime operating along the paleocoastline, with fluvial influence occurring predominantly during regression and tidal influence being enhanced during transgression (cf. Yoshida et al 2007;Ainsworth et al 2008Ainsworth et al , 2011. However, our interpretation also implies that preservation of facies tracts differed over the duration of a stratigraphic cycle, such that there is a strong preservational bias towards wave-dominated-shoreface deposits during regression.…”

Depositional evolution of a progradational to aggradational, mixed-influenced deltaic succession: Jurassic Tofte and Ile formations, southern Halten Terrace, offshore Norway

“…The absence of HCS in shallower and coarser grained parts of the succession may reflect the relatively coarse grain size, rather than a change in depositional processes (Dott and Bourgeois, 1982;Yoshida et al, 2007). The high degree of sorting in the coarser grained facies F2b-d is probably indicative of wave reworking (Yoshida et al, 2007;Hampson et al, 2011).…”

“…The homogenous mudstones (facies F4b and F4d) resemble fluid mudstones, indicating that wave action or tidal currents kept mud formed by salinity-induced mud flocculation in suspension (Ichaso and Dalrymple, 2009). The mature texture of well-sorted, medium-to coarse grained sandstones in facies F4d is attributed to sorting by waves, while the absence of HCS is attributed to the coarse grain size of available sediment (Yoshida et al, 2007). Cross-bedding in facies F4d may be due to dune migration in response to a combination of unidirectional fluvial, tidal and/or wave-generated longshore currents at the river mouth.…”

“…Therefore, the change from progradation in Phase II to aggradation in Phase III and the related decrease in sandstone grain size affects the preserved record of depositional process regimes. It is widely accepted that depositional systems become more tide-dominated during transgressions as the coastline rugosity increases and tidal currents are amplified in newly formed coastal embayments and drowned fluvial valleys Dalrymple, 2006;Yoshida et al, 2007). Also, coarse grained coastal deposits are more likely to be dominated by fluvial processes due to the proximity of these coastlines to the hinterland and the related more rapid response to changes in sediment M A N U S C R I P T…”

Introduction to the sedimentology of paralic reservoirs: recent advances

“…Despite this undoubted progress, the conference and the papers presented here amply demonstrate that progress continues in the sedimentological description of paralic reservoirs and that several recurring themes represent fruitful directions for future work: † The tectonostratigraphic context of paralic successions influences their subsidence patterns and preserved thickness, sediment input and routing, and potentially also the process regime (e.g. via suppression or amplification of waves and tides : Stride 1982;Yoshida et al 2007;Ainsworth et al 2008;Wells et al 2010). Spatial and temporal variations in tectonostratigraphic context may result in the development of subtle stratigraphic trapping configurations, and complex spatial patterns of reservoir distribution and character.…”