Sunday, December 14, 2008

Dracovenator by request: the age of the Elliot

Randy and Bill asked an apparently simple question that will require an involved answer. What age is the Elliot Formation? What follows is a detailed account of my thoughts that will probably get to technical for those without a geological bent.
Still here? Great I’ll try to make it worth your while. As I mentioned in the last post the Elliot Formation occurs near the top of the vast sedimentary pile that fills the famous Karoo Basin of South Africa. It underlies the Clarens Formation and overlies the Molteno Formation. Dating the formation is difficult as there is little to constrain it, there are no known ash or lava beds within it that have been radiometrically dated, and the formation lacks any marine microfossils or palynomorphs (spores and pollen) that could be used to date it. The upper constraint on its age comes from the Drakensburg lavas that overlie the Clarens Formation. These represent a rapid pulse of volcanism that is precisely dated to 183 million years ago, which places it at the Pliensbachian-Toarcian boundary of the Early Jurassic in the timescale of Gradstein et al. (2004). Thus the Elliot Formation cannot be any younger the Toarcian-Pliensbachian boundary. At the other end, the constraint on the maximum age of the Elliot Formation is much woollier. The Molteno Formation is said to have palynomorphs of Carnian age, but this is a thick unit and it is not known how much younger the upper parts of the Molteno Formation gets. Furthermore it is now know that terrestrial ‘Carnian’ deposits (e.g. the famous Ischigualasto Formation of South America) actually correlate with the latest Carnian to early Norian stages of the marine sequence. A Carnian/Norian age for the Molteno Formation is also supported by the vertebrate fauna of the Pebbly Arkose Formation of Zimbabwe which is presumed to be a lateral equivalent of the Molteno Formation. This fauna consists of a rhynchosaur and a primitive dinosaur that resembles Saturnalia (which comes from the ‘Ischigualastian’ of Brazil). Thus the Elliot Formation is unlikely to be any older than the Early Norian (about 225 million years old). However this is a huge spread of time, about 42 million years in fact, can we narrow it down any further? Before we examine this question we need to dispel a common model for the deposition of the Elliot Formation which is often portrayed in the literature. This is the model of continuous deposition. This figure (taken from Holzforster 2007) is typical.

In this diagram the Elliot Formation is portrayed as filling the entire block of time between the Molteno and Clarens Formation. However this is not realistic. Deposition in the Karoo Basin was controlled by subsidence in response to tectonic activity in the Cape Fold Belt. Tectonic activity is rarely, if ever, continuously sustained over tens of millions of years. Further the sediments themselves show evidence being deposited in discontinuous pulses. The lower and upper members of the Elliot Formation represent two such pulses. Although no angular unconformity or extensive erosional surface separates these two members there is evidence that there was a non-depositional gap between the deposition of these two units. Firstly we have the wholesale changeover in fauna between the two members. There is no known vertebrate species or genus that occurs in both units. There are also lithological differences that indicate that the style of rivers crossing the floodplain had changed, from sinuous, deep permanent streams to shallow emphemeral braided streams. The latter seems to be coupled with a more arid climate which is also betrayed by other indicators of aridity such as the development of extensive calcareous paleosols, deeply muckcracked overbank horizons, and evidence of floodplain denudation during flash-flood events. Other features such as palaeocurrent indicators suggest that the overall direction of drainage also changed as did the source of the sediment. These features could have suddenly ‘switched’ but a time gap allowing these features to change more gradually seems far more likely.
So if we accept two pulses of deposition what age constraints can we put on them? There is little doubt now that the first pulse was Late Triassic in age. I’m currently preparing a paper describing rauisuchians from the Elliot Formation. They have been reported before but this will be the first time the identification will be based on diagnostic derived characteristics. If we accept that rauisuchians went extinct at the end of the Triassic then these occurrences certainly place the lower Elliot in the Triassic. But where in the Triassic? Geology can help us a little here. According to the model of Catuneanu et al. 1998 and Bordy et al. 2004 the deposition in the part of the Karoo basin where the Elliot Formation crops out occurred during an offloading phase when the Cape Fold mountains were shedding sediment after a mountain building event. During the offloading phase the more distal part of the basin (where the Elliot Formation lies) sags after bulging upward, creating accommodation space for the sediment to collect in.


The reciporical flexural model for basin development, as applied to the deposition of the lower Elliot Formation. Part of a figure from Bordy et al. 2004.

Structural and metamorphic geologists date the end of the mountain building event that is believed to have immediately preceded the deposition of the lower Elliot Formation to 215 million years, give or take 3 million years. This puts the lower Elliot in the mid to late Norian, maybe even extendig into the Rhaetian (depending how long the offloading phase lasted after the mountain building finished) as Randy suggested. Whatever its age I’m willing to bet that the lower Elliot Formation is more or less equivalent to the vertebrate-bearing horizons of the Los Colorados Formation based on similarities of their sauropodomorph faunas. These similarities include Lessemsaurus and Antetonitrus which are very, very similar (though a few telling differences do keep them as separate taxa, e.g. the proportions of metatarsal I). Eucnemesaurus (ex Aliwalia) is also extremely similar to one of the taxa lurking under the label ‘Riojasaurus’.
OK so that’s the lower Elliot, what about the upper Elliot? Here we don’t have ahelping hand from geology. A pulse of mountain building after 215 million years has not been detected (indicating it was a minor event).
Faunally the upper Elliot Formation contains taxa that seem to indicate an Early Jurassic age (e.g. the crocodyliform Protosuchus and a diversity of ornithischians) but this is rather weak reasoning. Nevertheless I think an age younger than the Hettangian one usually assigned to the unit can be supported on the following grounds:
1. The same biozone (the Massospondylus Range Zone)can be found from the beginning of the upper Elliot through to the top of the Clarens. Fossils are extremely rare near the top of the Clarens but Billy De Klerk of the Albany museum has collected a couple of good Massospondylus skeletons, that if I recall correctly, come from near the top of the Clarens. Basically the fauna of the Clarens is a depleted subset of what you find in the upper Elliot (easily explained by the limited sample from the Clarens). The only possible faunal change is that the little trithelodontid cynodont, Pachygenelus, might be replaced by a different trithelodontid, Diarthrognathus, in the Clarens. This suggests to me that we aren't dealing with a large span of time. Dinosaur genera seem to turn over every five million years or so, thus we are probably dealing with a duration in this vicinity for the entire upper Elliot to Clarens sequence.
2. The deposition of the Clarens is terminated by a sudden and precisely dated volcanic event - the eruption of the Drakensburg lavas which is dated to 183 million years.
3. There is no evidence of a time gap between the Clarens Formation and the volcanic eruptions. Indeed there is evidence that one followed the other without a hiatus. For instance in some localities lava flows can be seen to have filled the interdunal spaces in the Clarens dune desert. Clarens deposition seems to have continued after the eruption of the first few flows in some places. A great example can be seen on the road between Barkly East and Rhodes in the Eastern Cape. Lastly there is evidence that the volcanism was begining during the deposition of the Clarens - as is shown by the Clarens filled crater reported by Holzforster (cited in my last post). It would be great to get a date for the pyroclastic flows that form the basal layers of this crater but sadly no-one seems to have done it yet - any young geochronologist looking for a project?
4. So if we accept that the end of the Clarens can be precisely dated to 183 million years AND we only allow a duration of 5 million years or so for the Massospondylus Range Zone then the begining of the upper Elliot Formatio dates to the Early Pliensbachian (about 188 million years). Perhaps we could allow a few million years to have elapsed right at the end of the Clarens, before the volcanic lava flows began spilling out over the basin, and allow a rather long duration for the Massospondylus and its cohabitants but that would still only take us down into the late part of the Sinemurian.

There you have it. My guess is that the lower Elliot is mid-late Norian, there is a hiatus of about 15 million years before the upper Elliot and Clarens which probably span most of the Pliensbachian. How can this be tested? Detrital zircons would be a wonderfull source of data - once again any young geochronologists out there looking for a project?

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.

Catuneanu O, Hancox PJ, Rubidge BS (1998) Reciporical flexural behaviour and contrasting stratigraphies: a new basin development model for the Karoo retroarc foreland system, South Africa. Basin Research 10: 417-439

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.

3 comments:

R.Butler said...

What is the dinosaur from the Pebbly Arkose Formation - is this the femur described by Raath (1996) or do you have more material?

Adam Yates said...

Just that bit of femur described by Raath I'm afraid. Max Langer (last time I discussed it with him) is adament that it is identical to Saturnalia and should be referred to it. However the mix of characters the proximal femur of Saturnalia is likely to be widespread amongst basal dinosaurs and probably near-dinosaur dinosauriforms as well.

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