February 17, 2019 | David F. Coppedge

Mudstones Form Rapidly

One of the most common sedimentary rocks can form a hundred times faster than previously thought.

In 2007, geologists learned that their theory for mudstones was incorrect. Mudstones—the most common sedimentary rocks—do not have to form in calm water, as tiny particles drift down the water column and collect on the bottom. Instead, particles can clump or flocculate in currents and settle out much more quickly (see 14 Dec 2007). Now, another model, based on experimental evidence, speeds up the process even more. This was just published in the AGU journal Geophysical Research Letters. Trower et al say,

Carbonate mudstones are key geochemical archives for past seawater chemistry, yet the origin of carbonate mud remains a subject of continued debate and uncertainty. Prevailing hypotheses have settled on two mechanisms: 1) direct precipitation in the water column, and 2) post‐mortem dispersal of mud‐sized algal skeletal components. However, both mechanisms conflict with geochemical observations in modern systems and are problematic in deep time. We tested the hypothesis that abrasion of carbonate sand during sediment transport might produce carbonate mud using laboratory experiments and a sediment transport model. We documented experimental mud production rates up to two orders of magnitude faster than rates estimated for other mechanisms. Combined with model calculations, these results illustrated that transport and abrasion of carbonate sand is a major source of carbonate mud.

Transport implies current flow. Abrasion implies an erosional process on pre-existing larger particles. If this new model is correct, it overthrows the picture of tiny silt particles sinking slowly in placid water. Mudstones, therefore, could form rapidly in dynamic conditions. Rates “two orders of magnitude faster” means, in plain English, up to 100 times faster than other mechanisms that had been proposed.

Eastern Grand Canyon shows rapid downcutting after sheet erosion
The Grand Canyon contains abundant sandstones and mudstones. The eastern Grand Canyon (Marble Canyon) shows rapid downcutting after sheet erosion.

The authors also suggest that mudstones form in “high energy” environments, such as hurricanes, storms and wave action in shoals. This is a far different environment than has been long believed and taught. Picture a flood geologist reading this:

Our experimental data demonstrated that abrasion of carbonate sand under transport conditions typical of high energy shoal environments produces carbonate mud at considerable rates. In many cases, experimental and model abrasion mud production rates are orders of magnitude faster than algal or precipitative mud production (Figure 3). This indicated that high energy transport of carbonate sand—on a shoal or beach, for example—can produce fluxes of mud comparable to these other mechanisms even over smaller areas of carbonate platforms. Intermittency of grain movement, in particular when grains are trapped within bedforms, plays a role in diminishing the effective abrasion rate of carbonate sand over long timescales (Davies et al., 1978; Trower et al., 2017). However, typical bed shear velocities in many high energy environments, like shoals, are persistently above the threshold for motion for the carbonate sand (Bathurst, 1975; Gonzalez & Eberli, 1997; Rankey et al., 2006), such that the production of mud by abrasion is not affected by the same intermittency factor as the abrasion of any individual sand grain. In quieter and/or deeper environments where fair weather conditions are below the threshold of motion, the production of mud through abrasion would be subject to the intermittency of bed shear velocities sufficient to transport sediment (e.g., storms). Large storms, like hurricanes, although occurring relatively infrequently, could generate mud at a fast rate for a short period of time by transporting grainy sediment in suspension over the area of an entire carbonate platform. For example, recent estimates suggest that the Great Bahama Bank is >65% grainy sediment (Harris et al., 2015). If this sediment is moved near the threshold of washload for one day by one hurricane per year, mud produced by abrasion during a hurricane could account for ~4% of the yearly mud production budget as estimated by Robbins et al. (1997) (Supporting Text S2). In contrast, fair-weather abrasion of the ~45% of the platform covered by “grainstone” facies (Harris et al., 2015) can account for 36% of the annual mud budget at our slowest experimental rate (Supporting Text S21). The true platform-averaged rate is likely to be higher considering the increased abrasion rate in more energetic settings like shoals and contributions from areas covered by “packstone” facies. Fair weather mud production by abrasion is therefore likely to be more significant than storm production over long timescales.

The paper at this time is listed as an “accepted article” by the American Geophysical Union, meaning that it could be subject to revision. Usually, those revisions are minor in papers that have been accepted and announced publicly.

Score another major point for creation geologists who have long doubted the slow-and-gradual processes used by materialists to claim vast periods of time are required for the rock record. Now, the hard work of mathematical modeling of the implications of these findings arrives. Let’s watch and see what comes of this paradigm shift.

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