Mud as Sand!

Mud is, as we all know, an important component in the sedimentary rock record, and in some successions, completely dominates the record. The picture below is from around Grand Junction, Colorado, USA, and shows a thick package of the Mancos Shale, deposited in the Western Interior Cretaceous Seaway. I don’t care who you are or where you’re from, that’s a lot of mud!



Anyway, mudrocks (encompassing silt- and clay-sized grain populations) occur across the depositional landscape, from floodplains to abyssal plains, and are easily the third-best grain size out there (the others, sand- and gravel-sized particles, are tied for first in the “best grain-size” category). And that grain-size holy trinity (mud, sand, gravel) represents the methodological hydrodynamic triumvirate that most sed/strat types deploy (almost casually) in the field: mud is deposited in low energy conditions, sand takes some energy to shift around, and gravels need quite the push to get moving through a system.

There has been some recent, ahem, erosion, of that venerable concept of quiet-water suspension fall-out of mud, however; recent work, both field-based and experimental, are beginning to show that muds may be a little more complicated than just the “fine-grained, laminated, organic rich --- 50 meters thick” nonsense that gets slapped down in your field book.

Schieber and Southard, 2009 pulled off a rather nice, simple flume experiment using mud (in the 10 – 20 micron range) in both fresh and saltwater. They were able, through careful use of time-lapse photography, observations on ripples that became attached to the flume wall, and by quickly draining the slurry from the flume, to capture a variety of ripple formsets made entirely out of mud! The data repository for the paper has some slick movies of the mud ripples forming and migrating, as well. The picture below is their Figure 1, on pg. 484. That there’s a ripple, a-yup.


Anyway, the neat-o thing about this ripple is the kind of sediment transport these muds are experiencing. It was possible, for example, that the muddy ripples just looked like sandy ripples, but were being deposited as fine-grained, turbid slurries, which would still be a fairly low energy condition for ripple formation, right? However, Shieber and Southard 2009 have shown that, in fact, the muds rapidly flocculate into silt and sand-sized particles, which are then transported at comparable velocities to regular ol’ sand and silt grains.

Of course, the implications are pretty obvious: previously interpreted quiet-water offshore muds, for example, might not be as quiet-water as we thought. And, post-depositional burial and compaction might result in the general obliteration of these ripple cross-laminations and bedforms from the record, and the superficial appearance of horizontally laminated mudrock. That really changes your interpretation of hydrodynamics, sedimentation rate, and how the mudrocks fit into whatever lithofacies association scheme you’ve cooked up for your rocks.

Wright and Marriott, 2007, came to the same general conclusions regarding mudrock in the Lower Old Red Sandstone (South Wales, UK). These mudrocks generally lack the stereotypical “fine-scale laminations” of most mudrocks; because of this, the assumption has been that these muddy units have been altered by soil formation. However, Wright and Marriott 2007 point out that these mudrocks are often interbedded with gravel-lens that have sharp contact (above and below) with the mudrock, that there are sharp truncations that separate mudrock from other mudrock, and that there are some faint, large scale architectural components associated with these mudrocks, reminiscent of accretion packages in fluvial macroforms. The picture below sums up, diagrammatically, their own field-based evidence for these interesting mud associations; it’s Wright and Marriott’s Figure 3, on pg. 95.



Wright and Marriott (2007) point out that this is sort of a big deal. The interpretation of muddy deposits as relatively continuous, flat-lying overbank deposits with lots of pedogenesis is FUNDAMENTALLY different from the interpretation of mudrock deposited as sand-sized aggregates within a channel complex. Hydrodynamics, Time, and Sedimentation Rate…there are big differences between these two models. And, from a practical side, if I’m depositing mud-rich plugs in an active channel as macroforms, that is going to really change fluid migration paths for hydrocarbons within the ostensibly permeable and porous channel complex sandstones.

Anyway, kind of a neat thing to think about, next time your haulin’ ass over the mudrocks to get up there at the obviously more interesting sandstones. Maybe these fine-grained bedload phases are more common than we realize?

WORKS CITED

Schieber, J., and Southard, J.B., 2009, Bedload transport of mud by floccule ripples – direct observation of ripple migration processes and their implications: Geology, v. 37, p. 483-486.

Wright, J.P., and Marriott, S.B., 2007, The dangers of taking mud for granted: lessons from Lower Old Red Sandstone dryland river systems of South Wales: Sedimentary Geology, v. 195, . 91-100