The turbidite-contourite-tidalite-baroclinite-hybridite problem: orthodoxy vs. empirical evidence behind the “Bouma Sequence”

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 contorts 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 contoured 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to introduce a simplistic “Bouma Sequence” in 1962, at a time of limited knowledge on deep-water processes, it is obsolete now in 2021 to apply this model to the rock record amid a wealth of new knowledge.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.

The high-frequency sea-level change in the aftermath of the Marinoan snowball Earth: Evidence from the Doushantuo formation in the northern margin of the Yangtze Craton, South China

“…Sedimentary structures in high-energy water environments are lacking, such as conglomerate with a clas-support fabric, cross-bedding, strong scouring surface or deformation structure. And its thickness distribution is stable, so we explain that it is the distal depositions of debris flow or turbidity current in the low-energy subtidal zone (Flügel, 2004;Shanmugam, 2000Shanmugam, , 2021.…”

“…Interpretation: the sandstone cycle sequence of this set develops a typical normal grain sequence, which may be an incomplete Baoma sequence (Ta, Tb, Te; Tb, Te) (Figure 3B, F). Then, the massive sand, sharp contacts, floating clast, normal graded sequence, and scouring structures are typical characteristics of simulated sandy debris flow deposition (Postma et al, 2014;Shanmugam, 2000Shanmugam, , 2021, which could be interpreted as sandy debris flow or turbidite deposits in subtidal facies. Sedimentary structures in high-energy water environments are lacking, such as conglomerate with a clas-support fabric, cross-bedding, strong scouring surface or deformation structure.…”

“…The lower layer of the cycle is LF4 grey and dark grey thick massive (30-120 cm) quartz fine sandstone, and the upper part of the cycle is composed of LF5 dark grey thin laminated (5-15 cm) siltstone and mudstone, forming a complete B-type fifth order cycle (Figure 7). These B type cycles constitute incomplete Boma sequence (Figure 3B, F), develop by low-density turbidite sequence and clastic flow deposits (Figure 3C, D) (Flügel, 2004;Postma et al, 2014;Shanmugam, 2000Shanmugam, , 2021.…”

Linking the impact of seismicity on palaeogeographic evolution and sedimentary architecture: A case study from Middle Jurassic succession of Spiti Himalaya

“…The absence of coarse-grained sediment in an anoxic zone infers depositional site below the wave and storm base. In the absence of wave and storm, the sharp scour base of massive calc-arenite beds suggests that these are the deposits of slope failure or turbidite (Bouma, 1962; Shanmugam, 2021). These massive beds may also be related to hybrid beds like turbidites-debrite couplets (Jamil et al 2021).…”

How is a turbidite actually deposited?

“…The experimental study indicates that the depositional discontinuities in a turbidite are primarily due to the autogenic energy fluctuations in natural-scale flows, rather than to sourcing conditions or shelf regime. The issue addressed by our study is of a crucial importance to the future of turbiditic research, as the empirical recognition of substantial flow fluctuations from turbidite outcrops ( 11 , 20 , 32 ) has brought turbidite sedimentology to the crossroads of two conceptual trends: (i) an uncritical interpretation of all such fluctuating turbidites as river-generated hyperpycnites ( 22 , 24 ) or (ii) a rejection of the Bouma sequence as obsolete, with an immediate questioning of the importance of turbidity currents in theoretical favor of other submarine bottom currents ( 40 , 41 ). In our view, the transport and deposition by turbidity current need to be better understood before the global ubiquity and geological importance of turbiditic sedimentation can possibly be disputed.…”