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Giant sediment-wave field and supercritical-flow bedforms in a Mississippian (Viséan) carbonate ramp, Tennessee-Kentucky, USA

Robert Handford - Colorado School of Mines

13/12/2023 4:00 pm (London)

Over the past two decades, high-resolution bathymetry surveys have revealed abundant, large-scale sediment waves on modern marine slopes worldwide. However, few ancient analogs have been recognized, possibly owing to the seismic-scale outcrops required for their identification. Sediment waves are large-scale bedforms, with wavelengths ranging from 100s of meters to several kilometers, and amplitudes between a few to 10s of meters. This study documents a giant, carbonate sediment-wave field in outcrops of a Mississippian Fort Payne Formation in Tennessee and south-central Kentucky. Sediment waves are present in clinothem foresets and bottomsets of a distally steepened, carbonate ramp across an area >20,000 km2 in water depths estimated to have ranged between 100-150 m. Furthermore, there are large- and small-scale sedimentary structures with characteristics indicating deposition from Froude-supercritical flows. Basin physiography, upwelling, density flows, and a prolific, heterozoan carbonate-factory, which produced prodigious amounts of crinoid skeletal grains in the clinothem foresets, were key factors in establishing and maintaining a sediment-wave depositional system. Steep slopes and deep water, which characterize many modern examples, were neither present nor required because of near-constant shedding of crinoid grains and basinward transport by sediment gravity flows and cascading density flows. Upper slope sediment-waves are 100-500 m wave length and include two types: (1) transverse-oriented shaley-crinoidal waves with abundant supercritical-flow bedforms and (2) sinuous to transverse-oriented mud waves crossed by furrows filled with crinoid rudstones interpreted as cyclic steps. Bottomset sediment waves include (1) dune-like bedforms with wave lengths and heights of ~50 m and ~10 m, respectively, and (2) compound sediment waves up to 700 m long and 50 m high (Fig. 2). They exhibit upslope and downslope asymmetry. Supercritical-flow bedforms are present in troughs and lee sides. Some sediment waves tend to “clean” and coarsen-upward from shaley carbonates near the base to carbonate with thin shale breaks towards the top. The presence of sediment waves in this ancient and well-known carbonate ramp, although surprising, cannot be unique. Their discovery should stimulate re-examination of other ancient carbonate slope and basin-floor systems.

Hybrid event beds in the context of ancient deep-lacustrine fan models: new insights from the early Cretaceous North Falkland Basin

Thomas Dodd - British Geological Survey

10/01/2024 4:00 pm (London)

Hybrid event beds (HEBs) are texturally and compositionally-diverse deposits preserved within deepwater settings. They are deposited by flows exhibiting ‘mixed behaviour’, forming complex, but ordered, successions of sandstone and mudstone, which are often challenging to predict in the subsurface. HEBs are well-documented in deep-marine settings, where they are well-known and characterised as effective fluid transmissibility barriers and baffles in reservoirs. By comparison, there are far-fewer studies of HEBs from deep-lacustrine settings (both ancient and modern), where their character and distribution remain relatively under-explored.
This presentation explores the analysis of 3D seismic data, wireline logs and core from a series of ancient deep-lacustrine fan systems in the early Cretaceous strata of the North Falkland Basin (Falkland Islands). From this analysis, deep-lacustrine HEBs are observed to comprise the same idealized sequence of the ‘H1–H5’ divisions as those reported in deep-marine settings. However, in this dataset H3 ‘debrite’ units can be sub-divided into ‘H3a–H3c’, based on: sharp or erosional intra-H3 contacts, bulk lithology, mud-content and discrete sedimentary textures. This study interprets the H3a–H3c sub-units as the products of multiple flow components formed through rearward longitudinal flow transformation processes, during the emplacement of a single HEB. HEBs are observed within lobe fringes, where flow types, energies, and transport mechanisms diversify as a result of flow transformation. The temporal context of HEB occurrences is considered in relation to stages of fan evolution, including: the initiation, growth (I), growth (II), by-pass, abandonment and termination phases. HEBs are observed in either the initiation phase, where flow interaction and erosion of initial substrates promoted mixed flow behaviour, or in the abandonment phase as facies belt retreated landward. Investigating these relationships provides a new understanding of HEB distribution and character in deep-lacustrine basins, and importantly the ancient deep-lacustrine subsurface.

Understanding Facies and Sequence Architecture in Sierra de Albarracín-Aliaga, Spain: 2023 IAS Summer School

2023 IAS Summer School leaders and participants

24/01/2024 4:00 pm (London)

Join us for a recap of the 2023 IAS Summer School held in Spain in October, 2023! This presentation will provide an overview of the summer school as well as scientific findings from the participants.