Analysis of Amalgamated Tidal Deposits in Incised-Valley Estuary and Abandoned Delta Plains

Abstract

Although the classic regressive-transgressive Brent Delta in the northern North Sea is well-studied, some potentially attractive reservoirs have been detected locally within the Etive, Ness and Tarbert formations, which contain anomalously thick (tens of meters), and as yet little-explained, cross-stratified sandstone units. The core and log data through these unit show an overall deepening upward trend in a strong tidal regime, with fluvial current and wave signals being subordinate. The most striking evidence for tidal currents is the orderly, stacked cross-stratified sandstone, which is up to 40∼70 m thick, and much too thick to be shoreface deposits and exhibiting very regular set thicknesses that would be unusual in a purely fluvial environment. Though the major components are subaqueous dunes, these amalgamated sand sheets, covering different areas of Viking Graben, are highly variable and display a distinct hierarchy. At the small scale (few m), stacked cross-strata sets are confined by low-angle master surfaces, together making a coarsening upward compound dune, the key building block. At a bigger scale (few 10s of m) the superimposed compound dunes gradually thin upwards, also show decreased fluvial influence and increased wave influence. They may therefore represent a single tidal bar in a transgressive setting. These bars are either distributed within channelized estuarine setting, where there are fluid mud and mud pebbles overlying the erosive base of the channel, or deposited within a free-standing setting containing large-scale crossbedding without significantly erosive bases. At the largest scale each tidal bar migrated landwards, and stacked into thick sand sheets of two types: incised-valley estuarine and abandoned delta-plain deposits, which are undergoing transgression. The former is characterized by very clean sandy tidal bars, which are underlain by muddier channelized tidal bars, whereas the latter is defined by muddier IHS dominated tidal bars underlain by sandier cleaner tidal bars that typically lack of IHS. The sedimentary differences between these two systems are mainly related to the sediment input system, particularly at the seaward. As the incised-valley estuary is generally lack of suspended sediment input in seaward, where might form the cleanest tidal deposits, whereas, the abandoned portions of delta-plain can receive suspended sediment from adjacent active distributaries system thus might present muddier tidal bars seaward.

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