Abstract
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, Río de la 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.
Highlights
1.1 The incentiveThe term “hyperpycnite” was first introduced by Mulder et al (2002) in an academic debate with me (Shanmugam 2002) on the origin of inverse grading by hyperpycnal flows
1.3 The problem Despite popular claims that (1) river flows transform into turbidity currents at plunge points near the shoreline (Kostic et al 2002; Lamb et al 2010), (2) hyperpycnal flows have the power to erode the seafloor and cause submarine canyons (Lamb et al 2010), (3) hyperpycnal flows develop an unique vertical sequence (Mulder et al 2003), and, (4) hyperpycnal flows are efficient in transporting sand across the shelf and can deliver sediments into the deep sea for developing submarine fans (Zavala and Arcuri 2016), our understanding of hyperpycnal flows and their deposits, in particular, in deep-water settings, is highly speculative
It is worth noting that facies models of both contourites (Stow and Faugères 2008) and hyperpycnites (Fig. 14a) exhibit inverse to normal grading in ascending order and both have internal hiatus (Shanmugam 2016b, his Fig. 9.19)
Summary
The term “hyperpycnite” (i.e., deposits of hyperpycnal flows) was first introduced by Mulder et al (2002) in an academic debate with me (Shanmugam 2002) on the origin of inverse grading by hyperpycnal flows. The following year, Mulder et al (2003) published their review paper with the introduction of the genetic facies model of hyperpycnites. I have been an ardent critic of all genetic facies models. Examples are: 1) “Is the turbidite facies association scheme valid for interpreting ancient submarine fan environment?” (Shanmugam et al 1985). 2) “The Bouma sequence and the turbidite mind set” (Shanmugam 1997). In continuing this trend, it is only logical to contribute this paper — “The hyperpycnite problem”
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