Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea

Numerous elongated mounds and channels were found at the top of the middle Miocene strata using 2D/3D seismic data in the Liwan Sag of Zhujiang River Mouth Basin(ZRMB)and the Beijiao Sag of Qiongdongnan Basin(QDNB).They occur at intervals and are rarely revealed by drilling wells in the deepwater areas.Origins of the mounds and channels are controversial and poorly understood.Based on an integrated analysis of the seismic attribute,palaeotectonics and palaeogeography,and drilling well encountering a mound,research results show that these mounds are dominantly distributed on the depression centres and/or slopes of the Liwan and Beijiao sags and developed in a bathyal sedimentary environment.In the Liwan and Beijiao sags,the mounds between channels(sub)parallel to one another are 1.0–1.5 km and 1.5–2.0 km wide,150–300 m and 150–200 m high,and extend straightly from west to east for 5–15 km and 8–20 km,respectively.Mounds and channels in the Liwan Sag are parallel with the regional slope.Mounds and channels in the Beijiao Sag,however,are at a small angle to the regional slope.According to internal geometry,texture and external morphology of mounds,the mounds in Beijiao Sag are divided into weak amplitude parallel reflections(mound type I),blank or chaotic reflections(mound type II),and internal mounded reflections(mound typeⅢ).The mounds in Liwan Sag,however,have the sole type,i.e.,mound type I.Mound type I originates from the incision of bottom currents and/or gravity flows.Mound type II results from gravity-driven sediments such as turbidite.Mound typeⅢis a result of deposition and incision of bottom currents simultaneously.The channels with high amplitude between mounds in the Beijiao and Liwan sags are a result of gravity-flow sediments and it is suggested they are filled by sandstone.Whereas channels with low-mediate amplitudes are filled by bottom-current sediments only in the Beijiao Sag,where they are dominantly composed of mudstone.This study provides new insights into the origins of the mounds and channels worldwide.

Episodic Carbonate Deposits on the Triassic Continental Slope in Southern China

Abstract Episodic carbonate deposits on the Triassic continental slope in southern China are mainly composed of gravity-flow limestones and contourite limestones. Gravity-flow limestones were well developed in the lower and middle Yangtze area in the Early Triassic and in the Yunnan-Guizhou-Guangxi area in the Early and Middle Triassic. Five fundamental types of gravity-flow limestones are recognized: slide limestone, debris-flow limestone, grain-flow limestone, turbidite limestone and rockfall limestone. They form six types of assemblage beds: slide-debris-flow limestones, slide-debris-flow-turbidite limestone, slide-debris-flow-grain-flow-turbidite limestone, rockfall-debris-flow limestone, debris-flow-turbidite limestone, and debris-flow-grain-flow-turbidite limestone. The first two were formed mainly in the Early Triassic slopes. The Middle Triassic slopes were characterized by widespread rockfall limestone. Growth faults, storms, earthquakes and oversteepened slopes are considered to be the probable triggers of the gravity flows.

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

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.

Review of research in internal-wave and internal-tide deposits of China:Discussion

This discussion of a review article by Gao et al. （2013）, published in the Journal of Palaeogeography （2（1）： 56-65）, is aimed at illustrating that interpretations of ten ancient examples in China and one in the central Appalachians （USA） as deep-water deposits of internal waves and internal tides are unsustainable. This critical assessment is based on an in-depth evaluation of oceanographic and sedimentologic data on internal waves and internal tides derived from 332 print and online published works during 1838-January 2013, which include empirical data on the physical characteristics of modern internal waves and internal tides from 51 regions of the world＇s oceans （Shanmugam, 2013a）. In addition, core and outcrop descriptions of deep-water strata from 35 case studies worldwide carried out by the author during 1974-2011, and a selected number of case studies published by other researchers are evaluated for identifying the sedimentological challenges associated with distinguishing types of bottom-current reworked sands in the ancient sedimentary record. The emerging conclusion is that any interpretation of ancient strata as deposits of internal waves and internal tides is premature.

150 Years(1872-2022)of research on deep-water processes,deposits,settings,triggers,and deformation:A difficult domain of progress,dichotomy,diversion,omission,and groupthink

In capturing a snapshot of 150 years(1872-2022)of research on deep-water processes,deposits,settings,triggers,and deformation,the following 22 topics are selected:(1)H.M.S.Challenger expedition(1872-1876):The discovering of the“Challenger Deep”by the H.M.S.Challenger in the Mariana Trench has been the single most important achievement in deep-water research.(2)Five pioneers amid 50 notable contributors:R.A.Bagnold,J.E.Sanders,G.D.Klein,F.P.Shepard,and C.D.Hollister.(3)Mass transport:Mass-transport deposits(MTD)are the most important deep-water facies in terms of volume,geohazards,and petroleum reservoirs.(4)Gravity flows:There are six basic types,namely(a)hyperpycnal flows,(b)turbidity currents,(c)debris flows,(d)liquefied/fluidized flows,(e)grain flows,and(f)thermohaline contour currents.Sandy debrites are the most important petroleum reservoir facies.Despite their popularity,turbidites are not an important reservoir facies.(5)Kelvin-Helmholtz(KH)waves:Turbidites,related to KH waves,with internal hiatus are not qualified to function as predictive facies models;nor are they fit for stratigraphic correlations.(6)High-density turbidity currents(HDTC):Misclassification of density-stratified gravity flows with laminar debris flows and turbulent turbidity currents as HDTC is flawed.Experimental generation of density-stratified gravity flows in flume studies has debunked the concept of HDTC.(7)Classification of turbidites:Contrary to the popular groupthink,turbidites are exclusive deposits of turbidity currents.(8)Bottom currents:The four basic types of deep-marine bottom currents are:(a)thermohaline-induced geotropic contour currents,(b)wind-driven bottom currents,(c)tide-driven bottom currents(mostly in submarine canyons),and(d)internal wave/tide-driven baroclinic currents.(9)Classification of contourites:Contrary to the popular groupthink,contourites are the exclusive deposits of thermohaline-induced geotropic contour currents.(10)Tidal currents in submarine canyons:Their velocity measurements have been the single most important achievement in deep-water process sedimentology.(11)Modern and ancient systems:There is a dichotomy between rare observations of turbidity currents in modern settings and overwhelming cases of interpretations of ancient turbidites in outcrops and cores.The reason is that turbidity currents are truly rare in nature,but the omnipotent presence of turbidites in the ancient rock record is the manifestation of groupthink induced by the turbidite facies model(i.e.,the Bouma Sequence).(12)Internal waves and tides:Despite their ubiquitous documentation in modern oceans,their ancient counterparts in outcrops are extremely rare.This is another dichotomy.(13)Hybrid flows:They are commonly developed by intersecting of down-slope gravity flows with along-slope contour currents.However,they are often misapplied to down-slope flow transformation of gravity flows.(14)Density(sediment)plumes:Deflected sediment plumes by wind forcing are common.Despite their importance in provenance studies,they are not adequately studied.(15)Hyperpycnal flows:They occur near the shoreline,next to the plunge point;but are of no relevance in deep-water environments.However,their importance in deep-marine settings is overhyped in recent literature.(16)Omission of erosional contact and internal hiatus:In order to promote genetic facies models that must not contain internal hiatuses,some researchers selectively omit internal hiatuses observed by the original authors.(17)Triggers of sediment failures:There are 22 types,but short-term triggers,such as earthquakes and meteorite impacts are more important than the conventional long-term trigger known as Eustasy.(18)Tsunami waves:Despite their sedimentologic importance,there are no reliable criteria for recognizing tsunami deposits in the ancient rock record.(19)Soft-Sediment Deformation Structures(SSDS):Although most SSDS are routinely interpreted as seismites,not all SSDS are caused by earthquakes.There are 10 other mechanisms,such as sediment loading,which can trigger liquefaction that can develop SSDS.(20)The Jackfork Group,Pennsylvanian,Ouachita Mountains,USA:Our reinterpretation of this classic North American flysch turbidites as MTD and bottomcurrent reworked sands has resulted in the longest academic debate with 42 printed pages in the AAPG Bulletin history since its founding in 1917.(21)Basin-floor fan model,Tertiary,North Sea:Our examination of nearly 12,000 ft(3658 m)of conventional core from Paleogene and Cretaceous deep-water sandstone reservoirs cored in 50 wells in 10 different areas or fields in the North Sea and Norwegian Sea reveals that these reservoirs are predominantly composed of MTDs,mainly sandy slumps and sandy debrites,and bottom-current reworked sands.Our core-seismic calibration debunked the conventional wisdom(groupthink)that basin-floor fans are composed of sandy turbidites in a sequence-stratigraphic framework.(22)Turbidite groupthink:A case study in illustrating how turbidite groupthink functions,without sound scientific methods,on the basis of published information on modern turbidity currents in Bute Inlet(fjord and estuary),British Columbia,Canada.This compendium is hybrid in composition between an atlas(with 108 figures)and a review article(with 348 references).The author admonishes scientists against deep-sea groupthink and provides a roadmap for future researchers by identifying potential topics for research involving density plumes,internal waves,tidal currents,tsunami waves,sediment deformation,and lowstand braid deltas.