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Zavala Journal of Palaeogeography (2019) 8:23 https://doi.org/10.1186/s42501-019-0037-3 Journal of Palaeogeography

ACADEMIC DISCUSSION Open Access The new knowledge is written on sedimentary rocks – a comment on Shanmugam’s paper “the hyperpycnite problem” Carlos Zavala1,2

Abstract In a recent contribution G. Shanmugam (2018) discusses and neglects the importance of hyperpycnal flows and hyperpycnites for the understanding of some 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 . Sustained hyperpycnal flows also provide a rational explanation for the origin of well sorted fine-grained massive with floating clasts, a deposit often wrongly related to sandy debris flows. Keywords: Hyperpycnites, , Sediment gravity flows

1 Introduction 2 Comments on Shanmugam (2018) paper “The In a recent paper G. Shanmugam (2018) relativized the hyperpycnite problem” importance of hyperpycnal flows as an important sedi- 2.1 Sand transport to deep waters by hyperpycnal flows ment transfer mechanism to associated lacustrine and Shanmugam (2018), page 199 right line: “There is not a marine basins. Controversially, hyperpycnal flows were single documented case of hyperpycnal flow, which is the first documented land derived sediment gravity flows transporting sand across the continental shelf, and sup- in lakes (Forel 1885) and in deep marine settings (Hee- plying sand beyond the modern shelf break”. zen et al. 1964). At present, our understanding of hyper- There is growing evidences provided by the study of pycnal flows and their related deposits (hyperpycnites) the discharges of Taiwan (Dadson et al. 2005)es- has been deeply improved due to a join effort on the pecially SW Taiwan rivers into the Gaoping Canyon (Liu study of ancient and recent deposits, complemented et al. 2006, 2012, 2016; Chiang and Yu 2008; Zhang et with detailed oceanographic observations, flume experi- al. 2018), the case of the Cap Timiris Canyon (Antobreh ments and mathematical modeling. and Krastel 2006), the Rhone fan (Mear 1984; Droz et al. Full discussion with Shanmugam (2018) will be the 2001; Tombo et al. 2015), the Var fan (Mulder scope of a forthcoming full paper. The objective of this et al. 2001; Khripounoff et al. 2009, 2012), the Hueneme short reply is to discuss some points observed by G. canyon in the Santa Monica Basin (Romans et al. 2009), Shanmugam (2018) concerning my recent paper (Zavala the very thick deep water hyperpycnites related to the and Arcuri 2016) focused on the recognition and inter- Missoula flood (Griggs et al. 1970; Brunner et al. 1999; pretation of ancient hyperpycnites. Normark and Reid 2003; Reid and Normark 2003), the Newport canyon in Southern California (Covault et al. 2010), the Geremeas in the Sardinian southern Correspondence: [email protected] margin (Meleddu et al. 2016), the Alsek Sea Valley in Al- 1Departamento de Geología, Universidad Nacional del Sur, San Juan 670, aska (Milliman et al. 1996), the failure of the Malpasset 8000 Bahía Blanca, Argentina Dam in the Mediterranean (Mulder et al. 2009), the Eel 2GCS Argentina SRL, Molina Campos 150, 8000 Bahía Blanca, Argentina

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zavala Journal of Palaeogeography (2019) 8:23 Page 2 of 8

submarine fan in the offshore of northern California below basin waters forming a hyperpycnal flow which (Paull et al. 2014), the Al Batha hyperpycnal system in can travel considerable distances carrying large volumes Oman (Bourget et al. 2010), the Santa Barbara Channel of sediment directly supplied from a river in flood. An in California (Warrick and Milliman 2003), and the underflow can only be considered as hyperpycnal (from Zaire (Congo) canyon (Heezen et al. 1964; Khripounoff the Greek ὑπέρ (hyper) meaning “over”, pycnal = density, et al. 2003; Savoye et al. 2009; Azpiroz-Zabala et al. from Greek: πυκνός (puknos) meaning “dense”) if it's 2017) among others. Khripounoff et al. (2003) docu- originated on land. The last excluded from the definition mented on March 8, 2001, a sediment-laden turbidity of “hyperpycnal flow” to all kinds of underflows gener- current in the Congo canyon travelling at 121 cm/sec at ated inside the basin, as the case of mass-transport com- 4000 m depth, 150 m above the channel floor, transport- plexes, intrabasinal turbidites, tempestites, cascadites, ing quartz-rich well sorted fine-grained sand (150– and turbulent flows derived from convective instability 200 μm) and large plant debris (wood, leaves, roots). (Parsons et al. 2007) or density stratification. Conse- This single flow was sustained for ten days. More re- quently hyperpycnal flows can only be formed at river cently Azpiroz-Zabala et al. (2017) recognized in the mouths. same turbidity system flows lasting an average of 6.7 days with peak velocities between 80 and 100 cm/sec. 2.3 Turbidity currents from plunging rivers Additionally in their review of recent sinuous Shanmugam (2018), page 205 right line: “No one has deep-water channels, Wynn et al. (2007) claimed that documented the transformation of river currents into tur- “Deep-water sinuous channels are dominantly fed by bidity currents at a shallow plunge point in modern mar- high-frequency or semi-continuous, low-density turbidity ine environments”. currents, some of which may be hyperpycnal at times of There is a lot of documentation about the hyperpycnal peak fluvial discharge”. More recently Zhang et al. origin of turbidites, both in shallow and deep waters (see (2018) showed the results of monitoring the turbidity comments on chapter 2.1 with references there). Add- currents in the Gaoping during 3.5 itionally, a very nice documentation of the 1954 Bonea years. The mooring system was located at a water depth River flood in Italy and its related hyperpycnal flow de- of 2104 m, 146 km far from the canyon head. They re- posits is also available (Budillon et al. 2005; Violante corded 20 turbidity currents directly attributed to peak 2009; Sacchi et al. 2009). Recently Katz et al. (2015) pub- river discharges during flood periods. The duration of lished a detailed observation of an actual hyperpycnal each individual flow ranged from a week up to a month. discharge in the Gulf of Aqaba (Red Sea). They shared a The associated interstitial water had high temperature very interesting video available online https://www.you- and less salinity respect to the ambient water, thus dem- tube.com/watch?v=4r9ndJ80_1Y onstrating that these turbidites were originated directly from hyperpycnal flow discharges. Zhang et al. (2018) 2.4 Hyperpycnal flow deposits and turbidites concluded that “These observations strongly suggest that Shanmugam (2018), page 205 right line: “Hyperpycnal hyperpycnal flow conditions associated with the river flows are defined solely on the basis of fluid density. floods during the typhoon season are the dominant Therefore, it is misleading to equate turbidity currents drivers of sediment redistribution in tectonically active with hyperpycnal flows”. and climatically disturbed areas such as Taiwan and its In our paper (Zavala and Arcuri 2016) we follow the connected submarine canyons, and support the link be- approach of Mutti and Ricci Lucchi (1972), Mutti (1992) tween upstream hyperpycnal flows and sustained turbid- and Mutti et al. (1999), considering as turbidites all ity currents in the deep sea”. those sediments deposited by sediment gravity flows and not strictly turbidity currents. According to this defin- 2.2 The origin of hyperpycnal flows ition, turbidites (intrabasinal or extrabasinal) include a Shanmugam (2018), page 199 right line: “Thus far, the broad spectrum of deposits ranging from matrix–or emphasis has been solely on river mouth hyperpycnal clast—supported conglomerates to graded mudstone flows (Mulder et al. 2003), thus ignoring density plumes beds. We absolutely agree with Mutti’s point of view, in other environments, such as open marine settings, far and we are convinced that this broad definition substan- away from the shoreline”. tially simplifies the discussion in a field in which sedi- Of course, the hyperpycnal condition can only be mentary processes, flow rheology, flow states are in most achieved at the coast. According to Bates (1953) a hyper- cases inferred from the careful analysis of sedimentary pycnal flow occurs when a subaerial (fluvial) system dis- rock bodies. In our paper (Zavala and Arcuri 2016)we charges a mixture of water and sediment with a bulk have clarified this topic in page 37 “Note that in this ap- density higher than that of the water in the reservoir. proach, the criterion of Mutti et al. (1999) is followed, When this situation occurs, the incoming flow sinks considering as turbidites the deposits of all types of Zavala Journal of Palaeogeography (2019) 8:23 Page 3 of 8

subaqueous sediment gravity flows independently if they of massive sands”. “The Ta division has also been attrib- are related or not to purely turbulent flows. Conse- uted to deposition from sandy debris flows (Shanmugam quently, in this work, the deposits of both Newtonian 1997)”. (fluid) and non-Newtonian (plastic) flows are included in In my paper (Zavala and Arcuri 2016) I don’t consider this category”. Clearly, we don’t equate turbidity currents the occurrence of massive sandstones as a diagnostic cri- (Newtonian turbulent flows) to hyperpycnal flows. teria for the recognition of hyperpycnites. Although Hyperpycnal flows can originate from different high fine-grained massive sandstones are a very common density flows, ranging from cohesive debris flows up to product of hyperpycnal flow deposits, only the presence low density turbidity currents (Zavala 2018). of entire leaves within fine-grained massive sandstones is considered a diagnostic feature that allows the recogni- 2.5 Coarse-grained deltas and hyperpycnal flows tion of hyperpycnal flow deposits (Zavala and Arcuri Shanmugam (2018), page 206 right line “At present, 2016, pp. 46). This is because the existence of entire coarse-grained deltas are totally ignored in studying leaves proves that extrabasinal materials were trans- hyperpycnal flows. As a consequence, all published exam- ported together with sand grains within a turbulent sus- ples of hyperpycnal flows are from fine-grained deltas, pension, and were then trapped during the progressive such as the Yellow in China”. collapse of the suspension cloud. Not true. Probably G. Shanmugam ignores one of the Of course, the accumulation of massive sandstones best known documentation of coarse- grained hyperpyc- has been for long in the past related to an “en masse” nal flows of different fan deltas in British Columbia, accumulation but this origin was largely speculative, Canada (Prior and Bornhold 1990; Bornhold and Prior, since most arguments were supported in the lack of 1990). Additionally, several additional examples of re- and the observation of floating cent bedload dominated hyperpycnal flows and their de- clasts which were considered as supported by an in- posits are available in Mulder and Chapron (2011). ternal flow cohesion. In his discussion about the validity of the high density 2.6 Hyperpycnal flows and the inverse to normally- paradigm, G. Shanmugam (1996) intro- graded sequence duces the concept of sandy debris flows for cohesive to Shanmugam (2018), page 217 left line “Importantly, no cohesionless debris flows supported by matrix strength, one has reproduced the entire inverse to normally-graded dispersive pressure and buoyant lift. G. Shanmugam also sequence with internal erosional surface (i.e., the hyper- pointed out that these sandy debris flows are common pycnite facies model) in laboratory flume experiments; in fine-grained sands with low to moderate mud content. nor has anyone documented this sequence from modern According to G. Shanmugam’s point of view: (I) almost settings. The conceptual hyperpycnite model exists only all kinds of massive or inversely graded clastic deposits in theory in publications, not in the real-world sediment- should be interpreted as accumulated by sandy debris ary record”. flows; (2) deposits containing floating outsized clasts can This is not true. Violante (2009), in his Fig. 10, pro- be produced only by debris flows; and (3) a traction car- vided a nice example- of an inverse to normally-graded pet developed beneath a turbulent flow should be interval in recent hyperpycnal flow deposit generated by regarded as a (Sohn 1997). the Bonea flood in 1954. Similar inverse to normally- G. Shanmugam (2015) proposed a sandy debris flow graded intervals have been documented in recent hyper- origin for fine-grained massive sandstones based on pycnal deposits from the Al Batha lobes by Bourget et flume experiments carried out by Marr et al. (2001). al. (2010), his Fig. 12. Hyperpycnites characterized These experiments were conducted in a 10 m long glass by couples of inverse-normal grading were well docu- flume with a slope ranging from 4.6° to 0°. Experimental mented in the Triassic Yanchang Formation (Ordos sediment gravity flows were primarily composed of clay, Basin, central China). Hyperpycnites developed not only well sorted fine-grained silica sand (110 μm), tap water, in sandstones (Yang et al., 2017a), but also in and a siliceous material produced as a residue from fine-grained sediments (Yang et al., 2017b). burning coal. These experiments show that, for these considered slopes, the generation of coherent gravity 2.7 Hyperpycnal flows and the origin of fine-grained flows with water contents ranging from 25 to 40 wt% re- massive sandstones quire the addition of some clay. The experiments were Shanmugam (2018), page 217 left line “Massive sand- carried out with bentonite (0.7 to 5 wt%) and kaolinite (7 stones, considered to be a recognition criteria for hyper- to 25 wt%). No experiments were performed to analyze pycnites (Steel et al. 2016; Zavala and Arcuri 2016), are the requirement of other common clays in lacustrine not unique to deposits of hyperpycnal flows. There are and marine settings, like chlorite, illite and smectite. Of alternative processes that can equally explain the origin course the final deposit was well sorted because they use Zavala Journal of Palaeogeography (2019) 8:23 Page 4 of 8

well sorted sand in the experiment. Nevertheless, flows debris flows because they can be produced under turbu- having a matrix strength can transport different grain lent flow conditions as long as the deposition of large size materials which are deposited “en masse” by cohe- clasts lags behind in a traction carpet.” sive freezing, giving the typical poorly sorted characteris- Main evidences that are against the interpretation tic of debris flow deposits. of a sandy debris flow origin for the example shown G. Shanmugam (2015) in his Fig. 15 (here reproduced in Fig. 1 include: in Fig. 1), provides an example of a core photograph of massive fine-grained showing a large floating 1) The deposit is clearly bimodal, suggesting the join mudstone clast with a planar clast fabric (Fig. 1), a typ- occurrence of two different depositional processes, ical bi-modal deposit. According to him these “evi- A) gradual collapse of suspended load (massive well dences” suggest a deposition from a laminar sandy sorted fine-grained sands and silt) transported within debris flow. The occurrence of mudstone clasts of differ- a diluted sustained turbulent flow as a consequence ent sizes and the sharp and irregular upper bedding con- of a progressive loss of flow capacity and B) bedload tact are interpreted as indicative of flow strength and (large and occasionally rounded clay clasts) of large deposition from cohesive freezing in a laminar plastic clast dragged by shear forces provided by the flow. According to me, this interpretation is absolutely overpassing long lived turbulent flow over the rising wrong. deposit-flow interface. Sohn (1997, p. 507) strongly criticizes the point of 2) Texture, sorting and sedimentary structures don’t view of G. Shanmugam about the deposition of support a debris flow origin for this interval. The fine-grained massive sandstones and floating clay clasts: deposit is mainly composed of well sorted fine- “.. large floating clasts cannot be foolproof evidence of grained sandstones, which suggest a highly selective

Fig. 1 Core photograph showing a typical example of a sandy debris flow deposit according to G. Shanmugam. Note the imbrication in the small clay clasts located close to the top. From Shanmugam (2015) Zavala Journal of Palaeogeography (2019) 8:23 Page 5 of 8

transportation mechanism like suspension of sand 4) Massive fine-grained sandstones are commonly grains in a long lasting low density turbulent flow. associated with levels of similar composition and Dispersive pressure in very fine grained sands is grain size, showing planar lamination and climbing not an efficient support mechanism because of ripples. The last suggests a common origin to these the negligible inertia of very small sand grains. deposits related to traction plus fallout of fine-grained Additionally there is no evidence of escaping sand sediments from a turbulent suspension, under pore fluid. different velocity and rates of sediment fallout (Zavala 3) The evident low clay matrix of the deposit is also and Pan 2018, their Fig. 15). against the interpretation of a debris flow origin. 5) Sandy debris flows cannot explain clast imbrication 4) The imbrication of the small clay clasts at the top within massive sandstones, since this structure indicates a flow moving from left to right. Imbrication suggests that clasts were free to roll as bedload at is very important since it suggest that clasts were the base of a progressively aggrading depositional transported as bedload at the base of a sustained surface (Zavala and Pan 2018, their Fig. 5) turbulent flow. Once again, not a debris flow. 2.8 Lofting rhythmites The above evidences suggest for these fine-grained Shanmugam (2018), page 217 right line: “Zavala and massive sandstones a gradual accumulation (Sanders Arcuri (2016, their Fig. 18), in justifying their criteria for 1965) from sustained low density turbidity currents with recognizing hyperpycnites, presented a core photograph associated bedload. showing rhythmites, which they called “lofting rhyth- Shanmugam (2012, 2015), p 138 considered that “Deb- mites”. The core photograph is from the modern Orinoco ris flows are capable of transporting gravel and Fan, off Orinoco Delta in Eastern Venezuela (their coarse-grained sand because of their inherent strength. In Fig. 15). Such rhythmites are common in deep-water contrast, turbidity currents cannot transport coarse sand tidal deposits (Cowan et al. 1998; Shanmugam 2003)”. and gravel in turbulent suspension”. The assumption The “deep water” tidal rhythmites studied by Cowan et that turbidity current cannot transport clasts can result al. (1998) are from the Muir Inlet, a macrotidal fjord in in dangerous oversimplifications. Alaska. These rhythmites are not equivalent to those de- Floating clasts in turbidites are not only possible but scribed in our case studies, since they were described in very common, because they are transported as bedload a core recovered from a water depth of 241 m, located (mostly creep and rolling) above a progressively raising less than 1 km far from the coast (Cowan et al. 1998, depositional surface (Postma et al. 1988; Kneller and Bran- their Fig. 1). These rhythmites are composed of silt-clay ney 1995; Sohn 1997; Branney and Kokelaar 2002;Man- couplets accumulated by a tide modulated suspension ville and White 2003). Flume experiments performed by settling from turbid plumes originating from meltwater Banerjee (1977), Arnott and Hand (1989) and Sumner et discharges, where black intervals are plankton (no plant al. (2008) demonstrated that the accumulation of massive remains were recognized). The example from the Ori- sandstones occur by the collapse of suspended load from noco Fan is located at a water depth of 1994 m, more waning dilute turbulent suspensions (1–2vol%ofparti- than 300 km far from the Orinoco littoral delta. In all cles) at bed aggradation rates in excess of 0.44 mm/s. case studies shown in our paper, lofting rhythmites are As a conclusion, the interpretation of fine-grained never associated with sedimentary structures indicative massive sandstones as accumulated by sandy debris of tidal action like sigmoidal cross bedding, and are flows creates more problems than it solves, because: always associated with massive and cross-bedded sand- stones suggesting an origin associated with sediment 1) Almost all thick fine-grained massive sandstones are gravity flows. The analysis of thin sections allows to tract relatively well sorted and have very little or no clay step by step the origin of this structure (Zavala et al. content (Zavala and Pan 2018, their Fig. 12). 2008, 2012), and conveniently explains the occurrence of 2) Slopes in inner shelf and in lakes usually are less than plant remains and mica at the surface lamina. 0.5°, which will not favor the movement of cohesive or poorly cohesive debris flows characterized by 2.9 Intrabasinal and extrabasinal turbidites matrix strength. Shanmugam (2018), page 220 left line: “Intrabasinal tur- 3) An accumulation from sandy debris flows cannot bidites are those with sediments derived locally from ad- adequately explain the facies recurrence between jacent shelf and got transported into the basin by massive and laminated sandstones commonly “classic” turbidity currents. In contrast, extrabasinal tur- observed in the field (Zavala and Pan 2018, their bidites are those with sediments derived from distant Fig. 15), and also the common association of land and delta and got transported into the basin by massive sandstones with low angle cross bedding. “flood-triggered” turbidity currents or hyperpycnal flows Zavala Journal of Palaeogeography (2019) 8:23 Page 6 of 8

(Fig. 16). In other words, large river-delta fed submar- Yellow River, he over enhanced the role of present depo- ine fans on passive continental margins, such as the sitional processes both in their characteristics and mag- Mississippi Fan and the Amazon Fan, would be nitudes to try to explain the record. classified as extrabasinal ”. The fact is that the application of a strict “uniformitar- This is not true. The distinction between intrabasinal ianism” to the understanding of fossil sedimentary and extrabasinal turbidites applies for single flows and successions can lead to serious mistakes, since it con- should not be generalized for entire systems. A deep sea strains past geologic rates and conditions to those of the fan can be internally composed of both intrabasinal and present. For a stratigrapher, it’s important to understand extrabasinal turbidites. Intrabasinal and extrabasinal if certain geological phenomena were possible in the turbidites display diagnostic characteristics that allow a geological record, and not only if these conditions are clear differentiation between them (Zavala and Arcuri achieved nowadays. The key point resides in carefully 2016). describing, reading, and interpreting sedimentary rocks in the field, since only the stratigraphic record contains 2.10 Sand and gravel transport by hyperpycnal flows both present and future knowledge. “Somewhere, Shanmugam (2018), page 221 left line: “However, hyper- something incredible is waiting to be known” Carl Sagan pycnal flows cannot be responsible for transporting gravel (1934–1996). and sand from the land, carrying them 10−100 km/s− 1 Abbreviations across the shelf, and delivering them to the deep sea. For cm: centimeter; cm/sec: centimeters for second; et al.: et alii; Fig: Figure; example, no one has ever documented by direct measure- Km: kilometer; m: meter; mm/s: millimeters for second; vol%: volume ments or observations of transport of gravel and sand by percent; wt%: weight percent; μm: micrometer hyperpycnal flows in suspension from the shoreline to the Acknowledgements deep sea in modern settings.” The author deeply acknowledges the comments and suggestions provided This is not true. The existence of deep water gravel by the Editor-in-Chief, Feng Zengzhao and two anonymous reviewers, which and pebbly sandstone deposits related to hyperpycnal substantially help in performing this manuscript. discharges of the Columbia river has been clearly docu- Authors contributions mented in the Cascadia Channel (Zuffa et al. 2000)in The author read and approved the final manuscript. cores located 200 km far from the coast and at a water Competing interests depth of 3820 m (Griggs et al. 1970; Normark and Reid The author declares that he has no competing interests. 2003). Individual pebbles are rounded to subrounded with diameters up to 4 cm. These gravel deposits contain Publisher’sNote a mixture of intrabasinal and extrabasinal components Springer Nature remains neutral with regard to jurisdictional claims in like molluscan shells, wood fragments, and different published maps and institutional affiliations. water depth foraminifera. 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