Wednesday, December 10, 2008

Thoughts from the field: Welcome to Lake Drumbo

I want to float some ideas based on some observations I've made in the field this year. So what have I been up to? Well firstly I've been poking my nose further south than I usually do. Most of the sites I've been working over the past half a decade are in the north end of the main Karoo Basin of South Africa. Here the dinosaur bearing Elliot Formation is relatively thin, probably because it lies over the solid Kaapval Craton, a big ancient continental block. This block probably prevented the basin floor from sagging too deeply during mountain building events off to the south (Tectonics is a VERY important factor controlling of deposition in the Karoo Basin). But down south you get off the Craton and the Elliot Formatin becomes thicker...over four times thicker. So with some money from Germany, and a collaborative team from Germany (led by Ollie Rauhut) and Great Britain, we began looking down south (mostly in the Eastern Cape Province). Despite the great thickness of sediment good outcrop is hard to find as the rainfall is quite high and vegetation rather thick.
Another rucurrent problem is figuring out where you are in the stratigraphy, these rocks don't come with labels!
So first a quick primer of the stratigraphy of the dinosaur bearing part of the Karoo.
The top of the sedimentary pile is usually referred to as the 'Stormberg Group' although this is not an officially recognised stratigraphic unit. The Stormberg Group consists of three formations: the Molteno, the Elliot and the Clarens. The Molteno is a series of coarse grained to conglomeratic sandstones, with fine grained siltstones, mudstones and coals in the south. It is rich in plant fossils (and some insects) but has not produced a single piece of bone - although there a some dinosaur tracks reputed to have come from this formation. The Elliot Formation marks the begining of the dinosaur body fossil record in South Africa. It is divided into two units: the upper and lower. At times it can be very difficult to determine in which unit you are in. The Elliot Formation is predominately made of red overbank muds and silts deposited on a humid (lower) to semi-arid (upper) floodplain. It would appear that the Elliot Formation covers quite a time range, with the lower member almost certainly being of Late Triassic age while the fauna of the upper member is very much Early Jurassic in aspect. The Clarens Formation consists of pale cream to white massive cliff-forming sandstones that are aeolian (wind blown) in origin. It records further aridification and the onset of a dune desert. Strangely the fauna doesn't seem to change much, if at all between the upper Elliot and the Clarens. You get pretty much the same taxa in the Clarens, just fewer specimens.
Now here is a parorama of the main valley wall on Upper Drumbo near Barkly East.

We excavated two dinosaur skeletons (Nelly and Charlie) from near the bottom of the Valley. You can see the cap of massive sanstone at the top of the peak in the centre (Castle Rock. This is clearly Clarens Formation. But where is the top of the Elliot Formation and where in the Elliot Formation do our dinosaurs come from?

At first glance the change in slope at the top of the valley seems to mirror the outcrop pattern of the boundary between the lower and upper Elliot Formations.

A colourised diagram of the outcrop of te Elliot and Clarens Formations. Note the change in slope between the two members of the the Elliot Formation. Modified from Bordy et al. 2004.
You can see this even better from a photograph taken from a higher vantage point looking across at Castle Rock, rather than up at it from the valley floor.

In this photo only the thick sandstone bench at the top of the valley can be seen. The smooth ramp-like slope leading up to Castle Rock is obvious. The mountains in the distance are made of the 2km thick pile of basalt that was extruded toward the end of the Liassic. You can read more about them here.
If this was the case then the upper Elliot Formation would form the smoother upper slopes leading to Castle Rock and our dinosaurs would be from the lower Elliot (hence Triassic). BUT one of them is almost certainly a Massospondylus (typical of the upper Elliot) and the sediments that enclose them are also typically upper Elliot in aspect. Perhaps everything from the base of Castle Rock to the valley floor is upper Elliot. Thats a verticle height of 210 m. The upper Elliot reaches thicknesses of 150 m down in the south, but 210m is a bit much. A few observations point me towards what I think is the answer. Firstly you will notice a thin bed of narrowly banded siltstone just below the big sandstones of the valley rim.

I've see a similar bed at the very base of the Clarens Formation in a number of locations (but not all). These are probably a series of playa lake deposits (about the only fossils you will find in them are little mussel shrimps or conchostracans) and they form a pretty good marker for the end of the Elliot.the smooth slope above the valley is not made of red overbank fines like the Elliot Formation. Further evidence that the upper slopes are actually within the Clarens Formation can be found if you actually climb up to them to take a look. The slope isn't made of red fluviatile mudstones, instead you will find thinly laminated pale creamy-grey shales. I've seen this lithology before - in small localised lenses of the Clarens Formation. They are interpreted as small interdune emphemaral pond, or playa lake deposits. However the thickness here (about 80 m) is, as far as I can find out, unprecedented. It seems to me that far from being a dune desert, this part of the world was host to a pretty large long lasting lake. Even more cool is that the same big package of shale can be found in near Blikana and at Rhodes. This may indicate a lake about 100 km across. Strange that I can find no mention of such an obvious feature in the literature. So what lived in or or around it (assuming my interpretation is correct? Bugger all I'm afraid. You won't find even scattered fish bones or scales. My guess is that it was very shallow, hypersaline and prone to frequent drying out. Much like Lake Eyre in the Tirari Desert of South Australia. Indeed given that it is surrounded by a dune desert and the northern section of the Lake is about the same size as the lake I'm suggesting, Lake Eyre makes a pretty good analogue.

Lake Eyre

However there IS a report of a large Lake in the Clarens Formation off to the South West of this deposit. However this lake is an altogether different kind of thing. Holzforster (2007) reports on lacustrine deposits in the Clarens Formation in the Exterem South West end of the outcrop of the Formation. However here the Clarens is incised down into the Elliot Formation, and the lake deposits are underlain be thick pyroclastic deposits. In other words this south-western lake was developed in a volcanic crater. Sadly no fossils have come from here either, - its a pity because a late Early Jurassic Liaoning-style lagerstatte would be sooo cool.

references

Bordy EM, Hancox PJ and Rubidge BS (2004) Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic), Karoo Supergroup, South Africa. South African Journal of Geology, 107: 395-410.

Holzforster F (2007) Lithology and depositional environments of the Lower Jurassic Clarens Formation in the eastern Cape, South Africa.South African Journal of Geology, 110: 543-560.

4 comments:

220mya said...

Awesome post! I was just reading up on Clarens geology for a part of my dissertation. Do you happen to have a PDF of the Holzforster reference?

What are your thoughts on the age of the Elliot Fm? If the Tr-Jr boundary is really between the upper and lower units, It seems to me that the most parsimonious ages for the Elliot is Rhaetian-Hettangian. As far as I can tell most authors consider the lower Elliot to be Norian in age, but then where does the Rhaetian fit in?

Randy

Bill Parker said...

Yes. Yes. What Randy asked....(his second and third questions).

great post.

Adam Yates said...

Randy,

Sorry no pdf. Technically our library is subscribed to the electronic version of SAJG but for some inexplicable reason just one recent issue is not available on their website and of course that's the issue with the paper. Grrr.

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