THE DYNAMIC EARTH: A BLOG ABOUT GEOLOGY AND THE EARTH SCIENCES

Saturday, August 6, 2011

Teredolites, substrates, and stratigraphy

Trace fossils are pretty rad in and of themselves, but they become especially awesome when they start telling us all sorts of sedimentologically and stratigraphically interesting stories. Particularly nifty for us soft rock types is the fact that trace fossils quite often record all sorts of neat information about substrate conditions. In terms of paleoenvironmental indicators, subtle variations in the consistency and nature of underlying sediment can have important implications for reconstructing ancient ecology or paleogeomorphology.

Perhaps more importantly, a huge amount of stratigraphic information is often wrapped up in bioturbated surfaces. The most famous example is seen in the Ophiomorpha vs. Thalassanoides dichotomy, where the same critter (a shrimp) produces two very different traces depending on if the sediment was soft and mucky or firm and compacted. This distinction is important stratigraphically, as a compacted substrate reflects erosion and exposure of previously buried sediment. On the basis of trace fossil assemblages, stratigraphers and paleoichnologists have identified a continuum of substrate types, generally ranging from softgrounds (think gloppy mud, or loose, wet sand), firmgrounds, and hardgrounds (think exposed coral reefs). Substrate consistency has all sorts of implications for picking out chronostratigraphically important surfaces.

By far, the bulk of ichnology/seds work has been undertaken in the marine realm. However, heading up the profile into marginal marine and coastal sediments holds some pretty exciting trace-to-rock relationships for us to interrogate. Perhaps niftiest of the traces found in the somewhat-drier-world of coastal rivers is the humble Teredolites.

The wild Teredolites looks like a stubby bulb, with a broad base that tapers up towards a shallow opening. They're commonly sand-filled, though muddy ones are not unheard of, and they often occur in clumps along surfaces. These little club-shaped excavations are dug out by cute little bivalves, colloquially known as "ship-worms" today. The picture below, seized from the USGS, gives you an idea of what they look like:



Because I'm not an oil geologist, I don't own a yacht, so my exposure to these little fellows is limited only to the stratigraphic record. And perhaps somewhat surprisingly, these guys are actually pretty damn common! In fact, some horizons are pretty much taken over by Teredolites, indicative of a considerable amount of woody (or, to use the fancy science word, xylic) substrates. Bromley et al. (1984) recognized that this was a fundamentally different condition from the traditional soft-firm-hardground spectrum, and helpfully named these "woodgrounds". Furthermore, Bromley et al. (1984) characterize the Teredolites ichnofacies as a suite of traces that uniquely identifies the occurrences of woodgrounds in the rock record. Importantly, a Teredolites-bored log that happened to get washed out onto a submarine fan is NOT an example of the Teredolites ichnofacies; you need to have a lot of Teredolites, in association with a substantial and in situ woody substrate to qualify.

How is a woodground different from the other "grounds" and, perhaps more importantly, who the hell cares? Well, for one thing, the presence of woody substrates is pretty distinct environmentally as well as ecologically. Natural accumulations of woody material are unique types of environments, and take on a special meaning in the stratigraphic record, where they're known as "coal". In order to get coal in the stratigraphic record, you have to have minimal clastic input, otherwise you just end up with a pile of sand (for instance) with disseminated bits of plant debris scattered throughout. In other words, coal can be diluted to nothing by the addition of a bunch of sediment.

Teredolites is commonly associated with coal deposits in the rock record. The picture below is from the Cretaceous Mesa Verde of Utah, and shows some classic little teredos in association with coal underneath (bumpy bits in the sand, above the hammer; the coal is the black stuff). The trace, then, was bored into the woody stuff beneath, then infilled with sand from a high sinuosity coastal river. So, these traces record the existence of a major coal accumulating swamp...



...of coarse, the presence of all that COAL tells you that too, right? So what's the big deal?

Well, Teredolites is also sometimes found within sediments, often in channeform deposits of ancient rivers, without any associated thick accumulations of coal. Sometimes there will be little chunks of coaly/woody material associated with the teredo traces, and sometimes these clasts can be quite common in the bases of channels. But, there's no big prominent beds of coal to be found anywhere! The picture below, also from the K of Utah, shows just this relationship, with teredo traces and wood/coal fragments within fluvial sandstones.



These Teredo and wood debris intraclasts are actually telling us a pretty cool story. We know, from their presence, that at some point, deposits of woody debris accumulated in the ancient landscape, providing homes for a bunch of cute little shipworms. In otherwords, at some point, the paleoenvironment of the setting in the picture above must have been something like the everglades today; lots of plants, lots of life, and importantly, very little clastic sediment moving around the landscape.

BUT, since these Teredolites-bored wood chunks are clasts in a higher energy sedimentary deposit, we know that those idyllic evergladesean must have come to an end when there was a dramatic and fundamental reorganization in the "plumbing" of the sedimentary system. Where there had previously been slow, steady accumulations of plant matter and very little sediment, there eventually came to be a much higher energy system dominated by erosion and reworking.

This is a pretty amazing reversal, if you think about it. We go from a swamp to a coastal river system, and the only evidence for all the time it took to accumulate the coal is found in the wood-debris lag deposits in some ancient rivers. It's a pretty dramatic story of landscape evolution and sediment routing, one that is luckily pointed out to us by some helpful trace fossils and clasts. The stratigraphic record is dominated by these major shifts in sediment accumulation and reworking, and sometimes the only evidence for a completely vanished landscape can be found in some subtle and seemingly insignificant bits and pieces scattered throughout the preserved stratigraphy.

WORKS CITED

Bromley, R.G., Pemberton, S.G., and Rahmani, R.A., 1984, A Cretaceous woodground: the Teredolites ichnofacies: Journal of Paleontology, v. 58, p. 488-498

3 comments:

Brian Romans said...

Glad to see you blogging again. Speaking of trace fossils, I posted a photo of an enigmatic one yesterday, which has resulted in some good discussion: http://www.wired.com/wiredscience/2011/08/what-are-these-patterns/

Eric said...

Thanks Brian! And that's a pretty slick trace on your blog...I'll add my vote to the "Feeding Trace" ID!

Geology Barriers said...

Glad to be able to read more on Teredolites & woodgrounds which I only recently came across...