Abstract
The underpinning problems of deep-water facies still remain unresolved. (1) The Tb, Tc, and Td divisions of the turbidite facies model, with traction structures, are an integral part of the “Bouma Sequence” (Ta, Tb, Tc, Td, Te). However, deposits of thermohaline contour currents, wind-driven bottom currents, deep-marine tidal currents, and baroclinic currents (internal waves and tides) also develop discrete rippled units, mimicking Tc. (2) The application of “cut-out” logic of sequences, which was originally introduced for the “Bouma Sequence”, with sharp basal contacts and sandy divisions containing well-developed traction structures, to muddy contourites with gradational basal contacts and an absence of well-developed traction structures is incongruent. (3) The presence of five internal divisions and hiatus in the muddy contourite facies model is in dispute. (4) Intersection of along slope contour currents with down slope sediment-gravity flows, triggering hybrid flows, also develops traction structures. (5) The comparison of genuine hybrid flows with down slope flow transformation of gravity flows is inconsistent with etymology of the term “hybrid”. (6) A reexamination of the Annot Sandstone at the Peira Cava type locality in SE France fails to validate either the orthodoxy of five internal divisions of the “Bouma Sequence” or their origin by turbidity currents. For example, the “Ta” division is composed of amalgamated units with inverse grading and floating mudstone clasts, suggesting a mass-transport deposit (MTD). The “Tb” and “Tc” divisions are composed of double mud layers and sigmoidal cross bedding, respectively, which suggest a tidalite origin. (7) Although it was reasonable to introduce a simplistic “Bouma Sequence” in 1962, at a time of limited knowledge on deep-water processes, it is obsolete now in 2021 to apply this model to the rock record amid a wealth of new knowledge. (8) The disconnect between 12 observed, but questionable, modern turbidity currents and over 10,000 interpreted ancient turbidites defies the doctrine of uniformitarianism. This disconnect is attributed to routine application of genetic facies models, without a pragmatic interpretation of empirical data. (9) A suggested solution to these problems is to interpret traction structures in the sedimentary record pragmatically on the basis of empirical field and experimental evidence, without any built-in bias using facies models, such as the “Bouma Sequence”. (10) Until reliable criteria are developed to distinguish traction structures of each type of bottom currents based on uniformitarianism, a general term “BCRS” (i.e., bottom-current reworked sands) is appropriate for deposits of all four kinds of bottom currents.
Highlights
The geologic importance of downslope and alongslope process and their deposits in deep-water environments was discussed in detail using 540 case studies by Shanmugam (2021a)
The main reason is that intervals of bottom-current reworked sands (BCRS) with traction structures may be interpreted as turbidites based on the application of the turbidite facies model (Bouma 1962), which is popularly known as the “Bouma Sequence”
10 Discussion According to Bouma (1962), traction structures are an integral part of the turbidite facies model, which has become known as the “Bouma Sequence”
Summary
The geologic importance of downslope and alongslope process and their deposits in deep-water environments was discussed in detail using 540 case studies by Shanmugam (2021a). One of the important lessons learned from these modern and ancient case studies was that intersection of downslope and alongslope processes on continental margins is common (Fig. 1). The main reason is that intervals of BCRS with traction structures may be interpreted as turbidites based on the application of the turbidite facies model (Bouma 1962), which is popularly known as the “Bouma Sequence”. The primary objective of this review is to identify key problem areas that pose direct challenges in distinguishing individual depositional facies. This review is based on global case studies from the North Atlantic, Mid-Pacific Mountains, Gulf of Cadiz, Gulf of Mexico, Offshore Nigeria, Bay of Bengal, and the Annot Sandstone in the Maritime Alps that served as the type locality for the development of the “Bouma Sequence”
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