Discussion of He et al. (2011, Geo-Marine Letters) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

Shanmugam, G.

doi:10.1007/s00367-011-0264-9

Cited by 15 publications

(8 citation statements)

References 46 publications

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“…To our knowledge, there have been no previous direct observations reported in the literature of the effects of internal tides and internal waves in contourite channel systems. Indeed, these data will also contribute to the ongoing debate surrounding the recognition of internal-wave and internal-tide deposits in ancient sedimentary successions (Gao et al, 1998;He et al, 2011;Shanmugam et al, 2011;Shanmugam, 2012Shanmugam, , 2013. Further documentation of shortand medium-term variability in the MOW bottom current system remains a key objective for future work.…”

Section: Resultsmentioning

confidence: 87%

The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction

et al. 2013

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Gulf of Cadiz bottom currents contourite channel sandy contourites internal tides/waves The Cadiz Contourite Channel is the largest and most prominent contourite channel in the middle slope of the Gulf of Cadiz, and is known to channelise the southern branch of the Lower Core of Mediterranean Outflow Water (MOW) as it flows westwards from the Gibraltar Gateway. The channel lies in water depth between 650 and 1500 m, is 150 km long, 2-12 km wide, up to 120 m deep, and broadly s-shaped in plan view. It has several associated subparallel marginal channels and shorter spillover channel segments. Its geometry is controlled by the interaction of a strong bottom current with the seafloor morphology, affected by neotectonic deformation and diapiric intrusion. Bottom photographs and dredge hauls reveal a channel floor shaped by high-energy flow, in places with bare rock, boulders and gravel, and elsewhere covered with sandy contourites. The rocky substrate and derived clasts are formed of authigenic iron-rich carbonates, testifying the high degree of fluid escape from adjacent diapiric ridges and mud volcanoes. The sandy substrate shows a wide range of current-induced bedforms including small, straight-crested ripples, large sinuous sand waves and dunes (wavelength 3.5-5 m, height 0.3-0.9 m), weak surface lineation on sands, and aligned gravel stringers and deep erosive scours around large boulders. Bedform orientation indicates flows directed to the south/south-west (main channel) and west (spillover channel), which can be related to MOW bottom currents, and current velocities that vary between about 0.2 and 0.8 m s −1 , even in the same channel location. However, current vane orientation was clearly responding, at least in part, to tidal effects and periodicity in the Gulf of Cadiz at the time the photographs were taken. Maximum current velocities are achieved by a combination of barotropic and internal tides (probably generated at the continental slope) that reinforce the normal MOW flow. In addition, meteorologically-induced internal waves with periods shorter than tidal ones may exert an even greater influence on current intensity, especially when they occur at times of sudden changes of meteorological forcing. This effect further influences MOW variability. In all cases, the funnelling effect of the Cadiz Channel amplifies tidal or meteorologically-induced bottom currents.

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Section: Resultsmentioning

confidence: 87%

The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction

et al. 2013

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“…baroclinites) (Gao and Eriksson 1991;Gao et al 1998;He et al 2011;Gao et al 2013). However, these interpretations and related issues were the subjects of lively debates (He et al 2012;Pomar et al 2013;Shanmugam 2012bShanmugam , 2013bShanmugam , 2014aShanmugam , 2014bGao et al 2014;Dunham and Saller 2014). At present, our understanding of the origin of traction structures in deep-water environments is in a state of flux.…”

Section: Academic Discussionmentioning

confidence: 99%

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

Shanmugam

2021

J. Palaeogeogr.

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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.

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“…We readily acknowledge that some issues raised by Shanmugam (2012) are pertinent and stimulating for future research in this domain. Other points are inconsequential, marginal, disputable, or redundant.…”

Section: Discussionmentioning

confidence: 96%

“…Within this context, Boyd et al (2008) did not claim that surface tidal currents could mobilize sand in deep-water areas, but merely suggested that this sand was transported by ebb tidal currents to the shelf break from where it moved downslope by gravity processes. Consequently, the findings of Boyd et al (2008), as cited by Shanmugam (2012), cannot be used to argue that surface tidal currents can extend into deep-water areas.…”

Section: Conceptual Clarification Of Internal and Surface Wavesmentioning

confidence: 87%

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Reply to the Discussion of He et al. (2011, Geo-Marine Letters) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

Jinxiong

Gao

et al. 2012

Geo-Mar Lett

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A point-by-point reply is here made to the discussion put forward by G. Shanmugam of our recent paper on internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China. We readily acknowledge that some issues raised by that author are pertinent. However, others are inconsequential, marginal, disputable, or redundant. The available evidence clearly supports our interpretation that the Middle Ordovician Xujiajuan Formation was deposited in a deep-water setting. Furthermore, we uphold that an internal-wave and internal-tide interpretation for these deposits is plausible in view of the occurrence of bidirectional cross-bedding and unidirectional cross-bedding dipping upslope in a deep-water environment. Also, we consider the request to provide conclusive evidence for the existence of a pycnocline in our stratigraphic record as unreasonable-it is self-evident that these data are currently not available, and the absence of proof is not proof of the contrary. Point-by-point reply

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Discussion of He et al. (2011, Geo-Marine Letters) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

Cited by 15 publications

References 46 publications

The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction

The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction

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

Reply to the Discussion of He et al. (2011, Geo-Marine Letters) Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

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