April 1, 2021

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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"

If you have ever played with sand on a beach you know what a sedimentary deposit is: an accumulation of particles (also known as grains or clasts) of variable size, resting at the end of a journey of erosion and transport. When hardened and cemented, the deposit earns the title of sedimentary rock. The surface of the earth is littered with such rocks, often forming piles that can be several kilometers thick. Since the beginning of modern geology, two differing schools of thought have been in contention about the dominant pathway responsible for these accumulations of sediment. One, the “inch by inch” school,[1] stresses the importance of a gradual buildup of sediment over time. The other, the “catastrophist” school, posits that much of what is preserved in the sedimentary record formed during rapid, episodic events, rising above the level of background sedimentary noise.[2]

The catastrophist approach is of obvious appeal for a person holding a worldview that embraces a recent creation and a historical global Flood. However, its viability requires physical mechanisms able to transport large volumes of sediment over long distances in relatively short amounts of time. Perhaps one of the most exciting breakthroughs in sedimentology that continues to unfold to this day was the recognition of the importance of one such mechanism: sediment gravity flows, and turbidity currents in particular.

Sediment gravity flows are mixtures of fluid and sediment kept in motion by gravitational energy along a sloping surface. Depending on the properties of the fluid and sediment, different flows can develop, eventually producing beds with distinctive and predictable internal organization and physical structures. Turbidity currents are one type of these flows, driven by a density contrast between sediment-laden fluid and ambient fluid, with grains kept in suspension by turbulence. The resulting deposits, called “turbidites,” typically consist of graded beds with a lower sandy partition overlain by a finer-grained partition.

The identification of turbidity currents as a major mechanism for the generation of graded bedding took place in the 1950s and represented a true revolution for sedimentary geology.[3] Up to that time, it had proven difficult to account for the origin of some thick sedimentary successions consisting of regularly interbedded sandstone and mudstone. Sands were perceived as a type of sediment restricted to shallow waters, and their tight association and alternation with mudstones bearing the signatures of deep water deposition was puzzling.[4] The turbidity current model effectively solved this enigma, providing a physical process capable of transporting sediment stored on the continental platforms all the way to abyssal depths. It also explained why coarser and finer particles were separated in turbidite bedding: they traveled at different depths within the same sediment plume and were selectively deposited at different times during the waning of the flow.

Technological advancements in oceanographic research are contributing much data that increase our understanding of turbidity currents. Notably, modern oceanic turbidity currents in continental margins have been “caught in the act,” with direct documentation of flow velocities of several meters per second, movement at the base of the flow of large objects weighing several hundred kilograms, and runoff distances of tens of kilometers.[5] Another relatively recent development includes recognition of a linkage between sustained fluvial discharge of sediment during river flooding events and initiation of submarine density gravity flows, directly integrating fluvial and deltaic systems with deep basinal deposits.[6] Finally, many mudstone units (and the finest-grained partition of turbidites), previously thought to represent settling from suspension in quiet waters, are being reinterpreted as forming under flowing currents. This makes mud-rich, lowdensity turbidity currents a likely mechanism for lateral transport of fine sediment, and even lime mud, to deep basinal areas.[7]

Turbidity currents are presently considered to be among the most important agents of transport on the earth.[8] No precise estimates exist on what percentage of sedimentary rocks and deposits consists of turbidites, but they are known to have built massive, kilometer-thick submarine fans, like the Bengal Fan, and represent an important target reservoir rock for hydrocarbon exploration, with examples preserved throughout the geologic column.[9]

If deposition by turbidity currents fits well the catastrophist approach, it is also true that there are other kinds of deposits (e.g., some carbonate buildups) for which I do not currently know a good catastrophist explanation. A certain amount of time between individual turbidity currents is also implied by the common occurrence of trace fossils at the top of turbiditic beds, indicative of the activity of living organisms on the substrate between flows. However, it is not a bad time to be a “catastrophist” geologist. The turbidity current revolution is just one of the examples showing a heightened sensitivity for and renewed recognition of the importance of rapid deposition. Granted, this “neocatastrophism” continues to favor a view of the earth as a system tending toward equilibrium, with occasional disturbances gradually readjusted. However, accepting a short time frame since Creation week does imply a major role for catastrophic processes, providing a firm compass to read the rocks and make a positive contribution in the modern scientific environment.

I find it fascinating that the Bible gives us an extensive account of the Flood, a global catastrophe that affected the earth. Taking this explicit reference seriously could lead us to a point of intersection between the physical and the revealed, where faith is affirmed through history.

NOTES

[1] NS Haile. The “piddling school” of geology. Nature 1997; 387:650.

[2] DV Ager. The nature of the stratigraphical record. London (UK): Mac-Millan; 1981, p. 122. 3.

[3] E Mutti, D Bernoulli, F Ricci Lucchi, R Tinterri. Turbidites and turbidity currents from alpine “flysch” to the exploration of continental margins. Sedimentology 2009; 56:267–318.

[4] Ibid.

[5] CK Paull, PJ Talling, KL Maier, D Parsons, J Xu, DW Caress, R Gwiazda, EM Lundsten, K Anderson, JP Barry, et al. Powerful turbidity currents driven by dense basal layers. Nature Communications 2018; 9(1):4114.

[6] Mutti et al., op. cit.

[7] LP Birgenheier, SA Moore. Carbonate mud deposited below storm wave base: a critical review. The Sedimentary Record 2018; 16(4):4–10.

[8] Paull et al., op. cit.

[9] T Nielsen, RD Shew, GS Steffens, JRJ Studlick. Atlas of deep-water outcrops. AAPG Studies in Geology 56. Tulsa (OK): Shell Exploration and Production and American Association of Petroleum Geologists, 2007, p. 504.

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Ronny Nalin is the director of the Geoscience Research Institute and an adjunct professor in the geology program at Loma Linda University. He earned his PhD in Earth Sciences at the University of Padova. His research interests revolve on stratigraphy and sedimentology of shallow marine deposits, with an emphasis on carbonate sedimentary products. His scientific contributions in these areas have been published in several peer-reviewed international journals.