Grade this graded bed (UPDATED)
...So let me ask you something, especially you sedimentary geologists...
This is a sample of the Martinsburg Formation, a clastic unit shed off the Taconian Orogeny and into the adjacent basin. It's exposed in the modern-day Shenandoah Valley, where it overlies Ordovician carbonates, and is overlain by the Silurian Massanutten Sandstone (which is correlative to the Tuscarora Formation). It's essentially a graywacke, showing rhythmic bedding traditionally interpreted as turbidite deposits. I collected this sample in the Shenandoah Valley a year and a half ago, on a camping trip with my family.
Then I put it on the NOVA rock saw and sliced it in half. This chunk went to my dad's back yard, where I ground it down and polished it up. The result is a decent look at the internal structure of the unit (you can click on it for higher resolution):
Note the pretty uniform weathering rind wrapping around the whole thing, like crust on a loaf of bread.
UPDATE: Woe is me; I forgot to include a sense of scale. The sample measures about 10 cm (~4 inches) on a side.
Here's the thing that gets me... While this portion ('upper' 2/3 of the sample) shows a clear fining-'upwards' sequence....
...this portion of the sample (lower 1/3) appears to show a coarsening-'upward' sequence:
In other words, in this 'graded bed,' the coarsest grains appear about 1/3 to 1/2 of the way 'up,' from 'bottom' to 'top'... What gives? This isn't part of the traditional Bouma sequence, is it? How does a bed like this form?
I'd appreciate any enlightenment you can offer.
This is a sample of the Martinsburg Formation, a clastic unit shed off the Taconian Orogeny and into the adjacent basin. It's exposed in the modern-day Shenandoah Valley, where it overlies Ordovician carbonates, and is overlain by the Silurian Massanutten Sandstone (which is correlative to the Tuscarora Formation). It's essentially a graywacke, showing rhythmic bedding traditionally interpreted as turbidite deposits. I collected this sample in the Shenandoah Valley a year and a half ago, on a camping trip with my family.
Then I put it on the NOVA rock saw and sliced it in half. This chunk went to my dad's back yard, where I ground it down and polished it up. The result is a decent look at the internal structure of the unit (you can click on it for higher resolution):
Note the pretty uniform weathering rind wrapping around the whole thing, like crust on a loaf of bread.
UPDATE: Woe is me; I forgot to include a sense of scale. The sample measures about 10 cm (~4 inches) on a side.
Here's the thing that gets me... While this portion ('upper' 2/3 of the sample) shows a clear fining-'upwards' sequence....
...this portion of the sample (lower 1/3) appears to show a coarsening-'upward' sequence:
In other words, in this 'graded bed,' the coarsest grains appear about 1/3 to 1/2 of the way 'up,' from 'bottom' to 'top'... What gives? This isn't part of the traditional Bouma sequence, is it? How does a bed like this form?
I'd appreciate any enlightenment you can offer.
Labels: primary structures, sediment, valley and ridge, weathering
12 Comments:
Is the lower, lightest zone part of the weathering rind? It still looks like there's some coarsening upwards above that, though.
Could it mark a gradual increase in energy level, not enough to scour, but to increase the grain size? Followed by a gradual, more long-term decrease in energy that we normally see in graded bedding.
Just a thought.
This sort of grading is actually one of the defining characteristics for identifying hyperpycnites. Hyperpycnites are lithified remains of hyperpycnal flows, which are turbulently-supported, negatively buoyant density undercurrents, generally associated with extreme discharge events from river mouths.
On-the-rocks, you are essentially correct about what hydrodynamically is going on here; the model for hyperpycnal initiation is that, as a river is experiencing a flood, it undergoes changes in discharge and sediment concentration. As the river flow is debauching into the basin (a "terminal" subaqueous sink, the dense current hugs the bottom and flows as a turbidity current. However, what distinguishes this kind of turbidity current from others is the duration: the river flood waxes and wanes over the course of hours to days, resulting in a current that has greater or lesser energy (and sediment capacity) concomitantly. That waxing and waning of the river discharge is therefore preserved as changes in grain size within a single depositional bed, generally in the coarsening- and then-fining upwards style. The gurus of hyperpycnal stuff are T. Mulder and Jim Syvitski; a good review paper (with lots of cites) is Mulder et al 2003, Marine and Petroleum Geology v. 20: http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V9Y-4B0SV1X-3-2B&_cdi=5911&_user=443835&_orig=search&_coverDate=09%2F30%2F2003&_sk=999799993&view=c&wchp=dGLzVtz-zSkWA&md5=7d4fc13b149bb2dd0c300f12e456479a&ie=/sdarticle.pdf
Of course, I'm not saying that this rock represents DEFINITIVELY a hyperpycnite...to do that I'd much rather see a detailed process sed-based strat column for the whole association. The whole "hyperpycnal" paradigm has gotten a little overused, lately, and is actually a pretty complicated, pretty confusing topic requiring (in my opinion) a lot more careful hydrodynamic work to actually parse out the details.
I'm sure Brian could say a lot more, though!
You want to be careful, after all, about subtle scouring relationships that might juxtapose two (or more) individual event beds, producing an apparent coarsening and then fining pattern. Or some sort of debris-flow transformation that has a turbulent plume over the top. Or any number of complicated flow hydrodynamic processes that might produce this sort of "hyperpycnal" type of sedimentation...
I promise I'll shut up now.
Firstly, Callan ... scale? :)
To me it looks like the most distinct coarsening-upward is really just the basal few cm (?) ... the bottom 1/4 of your last photograph. How thick is that?
The reason the thickness is important because there is a division of high-density turbidites (from Lowe, 1982) called a 'traction carpet' that shows a thin upward-coarsening division. But, this is typically only a cm thick max in sandstones. It looks like the CU section you are showing is a bit thicker.
In that case, what on-the-rocks and, more specifically, Eric are talking about could be in play.
So, at this point I'd like to see a precise scale on the bottom photo before making any "conclusion".
Very cool!
Oops... Sense of scale missing. Shame on me! Thanks for pointing that out, BrianR...
The sample measures about 10 cm on a side.
Yeah, so that distinct coarsening-upward part is ~1 cm ... this is maybe a bit too thick to be this 'traction carpet' division. Which makes me lean towards this part reflecting increasing energy (as on-the-rocks suggested) ... as for the hyperpycnal interpretation (as Eric mentioned), I'd want some more context and information. As Eric says, the recognition of hyperpycnites is in vogue right now ... which doesn't make it not true, but makes me not want to jump to that conclusion either. The problem with the hyperpycnite interpretation is that it's so specific ... it's incredibly difficult to demonstrate in ancient strata. But ... could be!
I agree, Brian - The problem with the hyperpycnal definition, which seems to always hinge on the coarsening then fining relationship, is that it does seem like a big leap from the waxing- then waning-flow strength (as on-the-rocks said) to the riverine sourced cause.
For instance, I wonder if you could get a quasi-steady state turbidity current from a long-lived coastal storm? Storm strengths wax and wane, and maybe that could result in these same sort of sorting patterns as seen in the hyperpycnal model?
Regardless, rad rock Callan. Flow hydrodynamics are always fun to talk about!
Eric says: "For instance, I wonder if you could get a quasi-steady state turbidity current from a long-lived coastal storm? Storm strengths wax and wane, and maybe that could result in these same sort of sorting patterns as seen in the hyperpycnal model?"
I think yes ... but, I don't have any references/data at my fingertips to back that up. Furthermore, I've always wondered why a waxing/waning flood hydrograph should necessarily be reproduced in the submarine gravity flow events.
Like you, I lean a little conservative on the hyperpycnal interpretation ... I think the important thing is to first conclude that the rocks do in fact represent waxing/waning energy fluctuations and THEN take the next step to interpret how that pattern was produced. Which is why I don't really like terms like "hyperpycnite", "seismo-turbidite", or any others that attach an initiation mechanism interpretation as the modifier.
Great Discussion! I learned a lot today!
Yeah, I'll second that, Lockwood.
I hesitated before I posted this, because I was worried I was missing something obvious about the sample. It's pleasing to me that, having taken the leap of admitting I don't know what's going on, that people who DO have a clue are informed and still aren't totally sure what's going on. Thanks VERY MUCH to everyone who has offered their expertise and insight today!
This has encouraged me to post similar "unknowns" here on the blog.
Eric (and others);
Thanks for the more detailed explanation of sediment/energy interactions with turbidites. It has been 20+ years since my last Sed. Petrology course at UTEP. I have to confess that I don't even remember terms such as "hyperpycnites" (and related derivatives) being used in class. And the only extensive turbidites that I have seen were of the Mississippian Tesnus Fm. in the Marathon, TX area.
Most of my experience with graded bedding has been in core samples of Cretaceous and early Tertiary fluvial/deltaic sediments on the inner Georgia Coastal Plain, while working with Paul Huddlestun.
In such settings, if there is a coarsening-upward sequence deposited, it would be susceptible to being scoured by the increasing energy levels during that particular event or by the next Bouma sequence. I recall truncated sequences being the rule, rather than the exception, in those units.
[Other than the core examinations, while working on the Georgia Coastal Plain, most of my work was in the field, not the lab.]
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