Facies model of a fine‐grained, tide‐dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert, Egypt

Crevasse subdeltas develop on modern river-dominated delta plains, and may be affected by the interaction of river currents and marine processes. However, their sedimentology and stratigraphic architecture is poorly constrained, leading to simplistic depositional models of delta-plain systems in the ancient record. Extensive exposures of the Middle Jurassic Lajas Formation permit the architecture, main stratigraphic surfaces, and lateral and vertical facies variations of crevasse subdelta deposits to be constrained. Lower-delta-plain successions studied in the Lajas Formation consist of up to 5-m-thick distributary channels and interdistributary-bay deposits, interpreted as crevasse subdeltas. Crevasse subdelta deposits consist of small-scale lenticular units (, 1-2 m thick) interpreted as crevasse channels and upward-coarsening and upward-thickening packages (, 2 m thick) with clinothems interpreted as crevasse mouth bars. These deposits preserve interbedding of coarser and finer sediments that are interpreted as river flood and interflood couplets associated with variations in river discharge. River flood beds are commonly structureless and erosionally based, and show little evidence of tidal action and brackish-water conditions. Interflood deposits show rhythmically distributed mudstone drapes, bimodality, and brackish trace fossils. This study highlights an important but largely undocumented component of interdistributary deposits consisting of tide-influenced, but strongly riverdominated, prograding depositional bodies. An implication is that some coarsening-upward, forward-accreting units previously interpreted from the rock record as interchannel ''tidal bars'' may instead represent minor mouth bars of tide-influenced crevasse subdeltas. Furthermore, present-day crevasse subdeltas are restricted to river-dominated delta systems that flow into semi-enclosed or enclosed seas and lakes with microtidal conditions and limited wave action, which is comparable to paleogeographic reconstructions for the Neuquén Basin during the Middle Jurassic.

Section: Understanding Of Ancient Upward-coarsening Deposits In Tidalmentioning

confidence: 94%

Stratigraphic Record of River-Dominated Crevasse Subdeltas With Tidal Influence (Lajas Formation, Argentina)

Gugliotta¹,

Flint²,

Hodgson³

et al. 2015

“…However, some recent studies, notably Willis et al (1999), Willis and Gabel (2001), Ainsworth (2003), Willis (2005), Ainsworth et al (2008), Plink-Björklund (2008), Ichaso and Dalrymple (2009), Steel et al (2012), and Legler et al (2013), document ancient successions that demonstrate significant tidal influence during regression and the development of seaward-stepping intervals. Recent studies of Holocene systems likewise showcase progradational settings reflecting marked tidal influence (e.g., Dashtgard et al 2009;Sisulak and Dashtgard 2012;Nanson et al 2012Nanson et al , 2013Massey et al 2014).…”

Section: Introductionmentioning

confidence: 95%

Architecture and Evolution of A Regressive, Tide-Influenced Marginal Marine Succession, Drumheller, Alberta, Canada

Ainsworth

Vakarelov

Lee

et al. 2015

Tide-dominated and tide-influenced clastic depositional environments are typically interpreted to be associated with landward-stepping or transgressive systems. Here, we present an example of an overall regressive succession dominated by high-frequency sequences that exhibit abundant sedimentary and ichnological evidence of tidal processes. These strata comprise the Campanian Bearpaw-Horseshoe Canyon Formation transition exposed near Drumheller, Alberta, Canada. The clastic marine to marginal-marine sediments were deposited in mixed-process (wave-, tide-, and fluvial-influenced) depositional environments along the western margin of a foreland basin. The deposits are subdivided into six relatively thin (10 m on average), high-frequency (, 140,000 years), transgressive-regressive (T-R) sequences (A to F).Outcrop sedimentary logs and hand-held gamma-ray profiles were correlated to subsurface data (4 cored wells and 75 wells with wireline logs) within an area of 22 km 3 16 km in order to generate a 3D geocellular computer model. This model along with paleocurrent data from outcrop were used to analyze the stratigraphic architecture and evolution of the succession.The evolution of the successive T-R sequences was mapped on a subregional scale using the 3D geocellular model. The orientations of the paleoshorelines rotate in a predictable manner during transgression and regression. Transgressive shorelines are oriented approximately S-N and rotate clockwise during regression to approximately SW-NE. The rotations are attributed to the paleogeography of the region, with the study area interpreted to sit on the western flank of a large structurally controlled embayment.The progradational high-frequency sequence set is dominated by tide-influenced strata and clearly demonstrates that tidal deposits can be preserved in both regressive as well as transgressive successions. Tidal influence in this regressive setting is attributed to shallow water depths on a wide shelf, which resulted in amplification of the tidal wave as it moved across the shelf. The embayed character of the coastline may also have augmented this effect. Tidal range is estimated to have been mesotidal to macrotidal. A modern partial analogue for the Horseshoe Canyon systems is identified from the tropical Mitchell River Delta in the Gulf of Carpentaria, Queensland, Australia. This delta system is analogous in terms of its tideinfluenced facies, mesotidal range, low-gradient shelf with accompanying shallow water depths, the mixed-process character of its shoreline systems, and its horizontal to falling shoreline trajectory.

“…Facies and stratigraphic architecture of IHS are typically complex (Thomas et al 1987;Makaske and Nap 1995;Dalrymple and Choi 2007;Hovikoski et al 2008;Hubbard et al 2011;Labrecque et al 2011;Fustic et al, 2012;Nardin et al 2012;Sisulak and Dashtgard 2012;Legler et al, 2013;Johnson and Dashtgard 2014), because multiple interrelated factors are involved in the genesis of IHS. More importantly, the intensity and frequency of the factors vary on different spatio-temporal scales (Johnson and Dashtgard 2014).…”

Section: Introductionmentioning

confidence: 98%

Morphodynamics of Tidal Channels In the Open Coast Macrotidal Flat, Southern Ganghwa Island In Gyeonggi Bay, West Coast of Korea

Choi

2015

Morphodynamics of intertidal channels were monitored in order to understand their implication on the architecture of inclined heterolithic stratification (IHS) in the open-coast Yeochari macro tidal flat on southern Ganghwa Island in Gyeonggi Bay, west coast of Korea. The tidal flat is divisible into narrow salt marshes in the upper intertidal zone, a concave-up upper to middle intertidal zone with small tidal creeks, and a channelized lower intertidal zone. Channels in the lower intertidal zone are 200-600 m wide and 1-2.5 m deep at bankful stage. They are sinuous in planform and have well-defined point bars and cutbanks. IHS is exposed along the cutbanks. High-precision leveling of the tidal channel bank unveiled that a point bar in the lower intertidal zone migrated about 400 m in 39 months. Channel migration is pronounced during the summer rainy season, when the point bar migrated as fast as 40 m per month, which led to rapid sediment accumulation of as much as 40 cm per month. In contrast, channel migration rate during winter is notably reduced, down to less than 1 m per month. Point-bar geometry alternates between a concave-up profile in summertime and a convex-up profile during the remainder of the year. Enhanced ebb currents during the rainy season, due to increased runoff discharge caused by heavy precipitation, accelerated point-bar migration. Remarkable rill erosion induced by heavy precipitation especially during low tide led to the rapid accumulation of sediment at the lower part of the point bar and the channel base, creating concave-up point-bar geometry. During the remainder of the year, the point bar retains a convex-up profile by continued sedimentation with insignificant rill erosion. Wave activity, particularly during winter, seems to facilitate deposition in the upper part of the point bar by providing more sediment into the channel. The present study illuminates the fact that the stratigraphic architecture of IHS of intertidal origin is controlled largely by monsoonal weather and to a lesser degree by tidal process even in a macrotidal environment. Careful examination of IHS geometry may hint at the depositional setting in terms of tidal frame (intertidal versus subtidal) and climate regime (monsoonal versus non-monsoonal).