Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocarbon reservoirs. Standard point‐bar models of how the internal sedimentology varies are based on the structure of small‐scale systems with little information about the largest complexes and how these might differ. Here a very large point bar (>25·0 m thick and 7·5 × 13·0 km across) on the Mississippi River (USA) was examined. The lithology and grain‐size characteristics at different parts of the point bar were determined by using a combination of coring and electrical conductivity logging. The data confirm that there is a general fining up‐section along most parts of the point bar, with a well‐defined transition from massive medium‐grained sands below about 9 to 11 m depth up into interbedded silts and fine–medium sand sediment (inclined heterolithic strata). There is also a poorly defined increase in sorting quality at the transition level. Massive medium sands are especially common in the region of the channel bend apex and regions upstream of that point. Downstream of the meander apex, there is much less evidence for fining up‐section. Finer sediment accumulated more readily after the establishment of a compound bar in the later stages of construction, at the terminal apex and in the bar tail. This work implies that the best reservoir sands are likely to be located in the centre of the point bar, deposited in a simple bar system. Reservoir quality decreases towards the bar edge. The early‐stage channel plug is largely composed of coarsening‐upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics.
Late Holocene Dead Sea rift‐margin strata reveal century‐scale sequences. Sequences are known to form at millennial timescales, but whether they can form at scales as short as centuries (seventh‐order, sensu Vail et al., 1991) was previously unconfirmed. This study maps lithofacies and sequence‐stratigraphic surfaces from a ca 35 m high outcrop of the post‐1500 ad part of the Holocene falling‐stage wedge, and adjacent subaerial environments, along the Nahal Darga of the western Dead Sea coast, using drone images. The study also produces the first maps of subaqueous delta environments from remotely operated vehicle photographs. Comparison of outcrop and remotely operated vehicle data show that delta foresets form by debris‐flow lobes accreting onto the delta face. Debris lobes increase in size with depth, have sorted cobble heads, and stack compensationally. Debris lobes dewater to form mostly erosive turbidity currents that cut channels and obstacle scours down slope. Topsets comprising fluvial, beach‐ridge and lagoonal deposits prograde over these foresets. Surfaces separating foreset and topset elements of this Gilbert‐delta system bind systems tracts and record downstep and upstep of the shoreface associated with base‐level rises and falls. These surfaces match known lake‐level changes in the Dead Sea in magnitude, timing and direction. They confirm Medieval (ca 600 to 1300 ad) and Modern (ca 1300 ad to Modern) century‐scale sequences. These microsequences form in the Dead Sea because this climate‐sensitive lake can generate metres‐scale lake‐level cycles on century timescales, and because depositional systems can, as shown here, reestablish sufficiently rapidly (≤101 years) to record century‐scale cycles with complete systems tracts. Although conditions for generating microsequences are optimal in the Dead Sea, they are not unique. These microsequences pose a challenge to sequence stratigraphy because they typically fall below seismic resolution, may geometrically resemble higher‐order composite sequences and complicate defining of anchor sequences within a sequence hierarchy.
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