Words of the Editor-in-Chief——some ideas about the comments and discussions of hyperpycnal flows and hyperpycnites

I basically agree with the viewpoints of Shanmugam(Journal of Palaeogeography 7(3):197-238,2018)and Zavala(Journal of Palaeogeog raphy 8(3):306-313,2019)who cited,refined and interpreted the definitions of hypopycnal flow,homopycnal flow and hyperpycnal flow.I appreciate two typical case studies of hyperpycnal flows induced by the Yellow River and Yangtze River,and the Gaoping River.The former is a normal type while the latter is catastrophic.They make up a complete knowledge about hyperpycnal flows and hyperpycnites.According to the interpretation of the word "hyperpycnal" from Greek to English,the "hypopycnal flow" should be "less density flow" or "lower density flow"("低密度流"),the "homopycnal flow" should be "equal density flow"("等密度流"),and the"hyperpycnal flow" should be "higher density flow" or "over density flow"("高密度流" or "超密度流").Some geologists called the "hypopycnal flow" as "异轻流"("abnormally light flow")and called the "hyperpycnal flow" as"异重流"("abnormally heavy flow").There are at least more than 10 names or terms about the "density flows" and the "deposits of density flows".It is a problem indeed.In addition,the density could be changed by salinity,temperature and pressure of water.Therefore,the term "density flow" may be problematic either.Another problem is that reliable and irrefutable identification markers of ancient heperpycnites are lacking.We should observe the policy of "A hundred flowers blossom and a hundred schools of thought contend" to discuss these problems and to promote progress and development of hyperpycnal flows and hyperpycnites.

Shallow-Water Origin of a Devonian Black Shale, Cleveland Shale Member (Ohio Shale), Northeastern Ohio, USA

Black shales are usually interpreted to require anoxic bottom waters and deeper water sedimentation. There has long been a debate about whether the Devonian Cleveland Shale Member of the Ohio Shale (CSM) was deposited in shallow- or deep-water depositional environments. This study looked at the CSM at 3 stratigraphic sections and 5 well cores in northeastern Ohio. The CSM mostly consists of sapropelite (interbedded carbonaceous black mudstones and gray calcareous claystones). The black and gray “shales” are rhythmically bedded at micro- (<1 cm thick), meso- (<10 cm thick) and macro-scales (10s of cm thick) and represent changes in organic matter content (ranging from 7% - 20% TOC). Three types of event layers are interbedded with the mudrocks: 1) tempestites, 2) proximal turbidites, and 3) hyperpycnites. Individual tempestites and turbidites are laterally continuous?≥35 km, while hyperpycnites are too thin (<1 cm) to trace laterally. Tempestites consist of hummocky stratified sandstones with groove casts and escape burrows overlain by planar laminated sandstones with wave ripples at the top. Tempestites average 13 cm thick, but can be amalgamated up to 45 cm thick, and are more common in the lower half of the unit. Turbidites are incomplete Bouma sequences that average 6 cm thick, show evidence of combined flow (“wave-modified turbidites”), and are more common toward the top of the unit. Hyperpycnites (density underflows from river discharge) consist of inverse-to-normal graded sandy or silty microlaminae that have been studied primarily by using petrography and SEM. Condensed sections in the CSM are probable firmgrounds with carbonate concretions, and indicate intervals of low sedimentation rates. The evidence shows that the CSM depositional environment was receiving siliciclastics from the northeast (e.g., Catskill delta), and that the coarser-grained clastic sediment was primarily transported as density underflows (turbidites and hyperpycnites). However, significant storm deposits (tempestites) within the CSM indicate erosion and redeposition occurred on a muddy clastic marine shelf at paleo-water depths less than storm-weather wave base (probably?≤50 m depth).

The new knowledge is written on sedimentary rocks——a comment on Shanmugam’s paper “the hyperpycnite problem”

In a recent contribution G. Shanmugam (2018) discusses and neglects the importance of hyperpycnal flows and hyperpycnites for the understanding of some sediment gravity flow deposits. For him, the hyperpycnal flow paradigm is strictly based on experimental and theoretical concepts, without the supporting empirical data from modern depositional systems. In this discussion I will demonstrate that G. Shanmugam overlooks growing evidences that support the importance of hyperpycnal flows in the accumulation of a huge volume of fossil clastic sediments. Sustained hyperpycnal flows also provide a rational explanation for the origin of well sorted fine-grained massive sandstones with floating clasts, a deposit often wrongly related to sandy debris flows.

Hyperpycnal (over density) flows and deposits

A hyperpycnal flow forms when a relatively dense land-derived gravity flow enters into a marine or lacustrine water reservoir. As a consequence of its excess of density, the incoming flow plunges in coastal areas, generating a highly dynamic and often long-lived dense underflow. Depending on the characteristics of the parent flow(flow duration and flow rheology) and basin salinity, the resulting deposits(hyperpycnites) can be very variable.According to flow duration, land-derived gravity flows can be classified into short-lived or long-lived flows. Shortlived gravity flows last for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, j?kulhlaups, etc. Long-lived gravity flows last for days,weeks or even months, and are mostly associated with medium-to large-size river discharges.Concerning the rheology of the incoming flow, hyperpycnal flows can be initiated by non-Newtonian(cohesive debris flows), Newtonian supercritical(lahars, hyperconcentrated flows, and concentrated flows) or Newtonian subcritical flows(pebbly, sandy or muddy sediment-laden turbulent flows). Once plunged, non-Newtonian and Newtonian supercritical flows require steep slopes to accelerate, allow the incorporation of ambient water and develop flow transformations in order to evolve into a turbidity current and travel further basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites. On the contrary, long-lived Newtonian subcritical flows are capable of transferring huge volumes of sediment, freshwater and organic matter far from the coast even along gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in pebbly and sandy hyperpycnal flows can result in lofting due to flow density reversal. Since the excess of density in muddy hyperpycnal flows is provided by silt-clay sediments in turbulent suspension, lofting is not possible even in marine/saline basins. Muddy hyperpycnal flows can also erode the basin bottom during their travel basinward,allowing the incorporation and transfer of intrabasinal sediments and organic matter. Long-lived hyperpycnal flow deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include(1) composite beds with gradual and recurrent changes in sediment grain-size and sedimentary structures,(2) mixture of extrabasinal and intrabasinal components,(3) internal and discontinuous erosional surfaces, and(4) lofting rhythmites in marine/saline basins.

Reply to discussions by Zavala(2019) and by Van Loon, Hüeneke, and Mulder(2019)on Shanmugam, G.(2018, Journal of Palaeogeography, 7(3): 197–238): ‘the hyperpycnite problem’

In this reply,I respond to 18 issues associated with comments made by Zavala(e.g.,inverse-to normally-graded sequence,origin of massive sands,experimental sandy debris flows,tidal rhythmites,facies models,etc.),and 10 issues associated with comments made by Van Loon et al.(e.g.,16 types of hyperpycnal flows,anthropogenic hyperpycnal flow,etc.).

The hyperpycnite problem

Sedimentologic, oceanographic, and hydraulic engineering publications on hyperpycnal flows claim that （1） river flows transform into turbidity currents at plunge points near the shoreline, （2） hyperpycnal flows have the power to erode the seafloor and cause submarine canyons, and, （3） hyperpycnal flows are efficient in transporting sand across the shelf and can deliver sediments into the deep sea for developing submarine fans. Importantly, these claims do have economic implications for the petroleum industry for predicting sandy reservoirs in deep-water petroleum exploration. However, these claims are based strictly on experimental or theoretical basis, without the supporting empirical data from modern depositional systems. Therefore, the primary purpose of this article is to rigorously evaluate the merits of these claims. A global evaluation of density plumes, based on 26 case studies （e.g., Yellow River, Yangtze River, Copper River, Hugli River （Ganges）, Guadalquivir River, Rio de ]a Plata Estuary, Zambezi River, among others）, suggests a complex variability in nature. Real-world examples show that density plumes （1） occur in six different environments （i.e., marine, lacustrine, estuarine, lagoon, bay, and reef）; （2） are composed of six different compositional materials （e.g., siliciclastic, calciclastic, planktonic, etc.）; （3） derive material from 11 different sources （e.g., river flood, tidal estuary, subglacial, etc.）; （4） are subjected to 15 different external controls （e.g., tidal shear fronts, ocean currents, cyclones, tsunamis, etc.）; and, （5） exhibit 24 configurations （e.g., lobate, coalescing, linear, swirly, U-Turn, anastomosing, etc.）. Major problem areas are： （1） There are at least 16 types of hyperpycnal flows （e.g., density flow, underflow, high-density hyperpycnal plume, high-turbid mass flow, tide-modulated hyperpycnal flow, cyclone-induced hyperpycnal turbidity current, multi-layer hyperpycnal flows, etc.）, without an underpinning principle of fluid dynamics. （2） The basic tenet that river currents transform into turbidity currents at plunge points near the shoreline is based on an experiment that used fresh tap water as a standing body. In attempting to understand all density plumes, such an experimental result is inapplicable to marine waters （sea or ocean） with a higher density due to salt content. （3） Published velocity measurements from the Yellow River mouth, a classic area, are of tidal currents, not of hyperpycnal flows. Importantly, the presence of tidal shear front at the Yellow River mouth limits seaward transport of sediments. （4） Despite its popularity, the hyperpycnite facies model has not been validated by laboratory experiments or by real-world empirical field data from modern settings. （5） The presence of an erosional surface within a single hyperpycnite depositional unit is antithetical to the basic principles of stratigraphy. （6） The hypothetical model of ＂extrabasinal turbidites＂, deposited by river-flood triggered hyperpycnal flows, is untenable. This is because high-density turbidity currents, which serve as the conceptual basis for the model, have never been documented in the world＇s oceans. （7） Although plant remains are considered a criterion for recognizing hyperpycnites, the ＂Type 1＂ shelf-incising canyons having heads with connection to a major river or estuarine system could serve as a conduit for transporting plant remains by other processes, such as tidal currents. （8） Genuine hyperpycnal flows are feeble and muddy by nature, and they are confined to the inner shelf in modern settings. （9） Distinguishing criteria of ancient hyperpycnites from turbidites or contourites are muddled. （10） After 65 years of research since Bates （AAPG Bulletin 37： 2119-2162, 1953）, our understanding of hyperpycnal flows and their deposits is still incomplete and without clarity.

Deltas:a new classification expanding Bates's concepts

Deltas constitute complex depositional systems formed when a land-derived gravity-flow(carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic sedimentary environments has been oversimplified by geoscientists over the years, considering just littoral deltas as the unique possible type of delta in natural systems. Nevertheless, a rational analysis suggests that deltas can be much more complex. In fact, the characteristics of deltaic deposits will depend on a complex interplay between the bulk density of the incoming flow and the salinity of the receiving water body. This paper explores the natural conditions of deltaic sedimentation according to different density contrasts. The rational analysis of deltaic systems allows to recognize three main fields for deltaic sedimentation, corresponding to(1) hypopycnal(2) homopycnal and(3) hyperpycnal delta settings. The hypopycnal delta field represents the situation when the bulk density of the incoming flow is lower than the density of the water in the basin. According to the salinity of the receiving water body, three different types of hypopycnal littoral deltas are recognized: hypersaline littoral deltas(HSLD), marine littoral deltas(MLD), and brackish littoral deltas(BLD). The basin salinity will determine the capacity of the delta for producing effective buoyant plumes, and consequently the characteristics and extension of prodelta deposits.Homopycnal littoral deltas(HOLD) form when the density of the incoming flow is roughly similar to the density of the water in the receiving basin. This situation is typical of clean bedload-dominated rivers entering freshwater lakes. Delta front deposits are dominated by sediment avalanches. Typical fallout prodelta deposits are absent or poorly developed since no buoyant plumes are generated. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the receiving basin. The interaction between flow type,flow density(due to the concentration of suspended sediments) and basin salinity defines three types of deltas,corresponding to hyperpycnal littoral deltas(HLD), hyperpycnal subaqueous deltas(HSD), and hyperpycnal fan deltas(HFD). Hyperpycnal littoral deltas are low-gradient shallow-water deltas formed when dirty rivers enter into brackish or normal-salinity marine basins, typically in wave or tide-dominated epicontinental seas or brackish lakes.Hyperpycnal subaqueous deltas represent the most common type of hyperpycnal delta, with channels and lobes generated in marine and lacustrine settings during long-lasting sediment-laden river-flood discharges. Finally,hyperpycnal fan deltas are subaqueous delta systems generated on high-gradient lacustrine or marine settings by episodic high-density fluvial discharges.