Dinosaur Dreaming

2001 Taphonomic update

Doris Villiers

As in previous years and years to come one of the main activities undertaken during the field trip is the recording of the extent, thickness and the change in appearance of each individual layer of sediment within the ancient riverbed that we are excavating. The results obtained confirm that it is important to record any minor and major changes during every stage of excavation.

In most areas we are working, we find three different pulses, but their appearance has changed in one case this year. Each of the pulses mentioned is made up of two different sections. The top part consists of pure sandstone. The section underneath is made up of predominantly clast-supported pebble (4 to 64 millimetres) sized conglomerate, with the pebble size ranging mainly between 2 and 10 millimetres. This conglomerate, however, has changed in the uppermost pulse.

Hints of these variations were present in last year's excavation area, but at that stage it was not clear that they were what seems now to be a permanent feature. Instead of being a mainly clast-supported conglomerate, the basal part of the last pulse has now been replaced by a layer of large mud clasts, which can consist of several clasts stacked on top of each other, but is mainly represented by a single layer of clasts. Though they usually are still within the pebbles size range, some of them are larger (up to 70 millimetres) and therefore must be called cobbles (64 to 256 millimetre, large fragments).

In some areas imbrication can be observed as well. To understand what imbrication means, imagine a modern alpine riverbed with an abundance of river pebbles and a little water flowing in between. Have you ever noticed that these pebbles usually are overlapping each other just like books on a bookshelf that have tipped over to one side? Well, that's exactly what imbrication is. Due to the current action under high flow velocity the pebbles become aligned in one direction. The resulting overlapping is an indication of the direction of the water flow. (See figure below).

Further changes are again visible in the conglomeratic layer of the second pulse. The predominantly clast-supported sediments are now often split up into several small subpulses, conglomerate and sandstone alternating several times within a few centimetres of deposits.

These changes might very well be caused by a change of the digging position within the ancient riverbed. So far, we have been excavating in the deeper parts of the river channel, which explains the more or less uniformity and thickness of the individual layers. As we get closer to the edges of the river we get a lower energy regime, resulting in thinner layers (caused by a modest material transport and possibly resulting in a change of appearance as is the case in the last pulse).

Palynology & Dendrology

In the last few years palynology samples have been taken from and around the cryoturbation close to the excavation site, a further cryoturbation north of the site and some areas around the volcanic dyke.

Cryoturbations are periglacial structures that are associated with seasonal freezing and thawing of soils. They only occur within a certain temperature range of round the 0° Celsius mark. However, so far, a third cryoturbation whose location was roughly known could never be located exactly. This year we got lucky. The sand in the very area of this cryoturbation was low, and it seemed furthermore that a storm and/or a big wave had washed the debris off the cliff face revealing its treasure underneath. Now that the precise position of the site is known samples can finally be taken.

You might ask why this task has not been targeted so far. Although the sand was low at the site of this third permafrost feature, the rest of the area was buried. The problem with pollen samples is that they can only be taken in very fine-grained material such as mudstone and best from carbonaceous beds. The reason being that the grains of pollen and spores are so fine that they get washed away in any higher energy environment which is represented by, for example, the sandstones that we find at the site.

The area that needs to be sampled is along a quite large stretch of beach. Mudstones and siltstones, the bearers of pollen and spores are here interbedded by often very thick layers of sandstone that do not have to be sampled. In order to maintain a good overview of what has been and what still needs to be sampled it is best to have the entire area - or if that is not possible at least a large stretch of it - exposed, so that it can be certain that all samples necessary have been taken and that no doubling up has occurred.

It is going to be a time-consuming exercise and patience might be stretched but in the end it might hold a very interesting outcome. It might very well be that the species diversity and the amount of individual grains found within the sediments of samples taken around the cryoturbation is very different to the ones in the long reaches between the periglacial features. A result like this could then be interpreted as at least a slight change in climate with the cryoturbation sections representing the colder stretches and the areas in between as the warmer ones. Furthermore, flora tolerant to colder climate could be identified as well.

Finding the third cryoturbation was not the only time we had luck, we also found one more fossilized tree trunk on the shore platform. It seems that the sand had shifted and exposed the fossil. Samples were taken with the help of Raúl and thin sections are being made of that to determine what kind of tree we are dealing with. This again shows that it is important to keep on going into the field in order to get more data and to continuously look for more clues, such as looking for the rings and structure of early and late wood.

Prospect for 2002

Next field season the excavation area will be extended further towards the cliffs (at a safe distance, of course). As mentioned before, it seems we are getting closer to the edge of the river, where water energy diminished. Lower water energy means higher and better preservation potential for fossil material, as bones would have been less tumbled and therefore less damaged in the water and sediment mixture. This also means that possibly very small and fragile bones, such as the ones of our very tiny mammals, could have been preserved in this area. And if we are ever going to find any articulated skeletons this would be the best place to find them. As again lower energy environment would not have had the strength to rip carcasses apart.

Furthermore, it might be a good idea to actually excavate down to the lowest conglomerate. After a few trial seasons we had, in the last few years, decided against it. That layer seemed to produce fossils of mainly turtle and fish (the animals actually living in the river) but hardly any other remains. If the theory of being close to the edge of the river is correct, it is very likely that we'll find not only smaller material, but also a different assemblage fauna. The closer to the edge we get the more the deposits should be influenced by what happened and lived around the shores of the river. We could potentially find fauna that lived and fed along its edges.

The reason why we do not find an abundance of other fauna in the lowermost pulse could, for example, very well be caused by the pulse that formed the deposit not being strong or big enough to actually go over the edges of the river and onto the flood plain, which means hardly any bone material of terrestrial fauna would have reached the channel. Alternatively, if the pulse was weak and only strong enough to reach over the edges of the river it would have, once it had done so, been too minute to actually flow back into the river let alone be strong enough to move any bone material. At this stage these are preliminary ideas and more work needs to be done to confirm or dispel them. To find out what really happened, however, we should try to do some more excavation within that first pulse.

As always at this stage, I have to thank everyone who has made my life of taking measurements and samples easier. Lesley, Mary, Nick, Norman, Cilla and Nicola of course, for halting the excavation process and doing my measurements for and with me and basically everyone else for collecting and keeping unusual items such as different coloured sediment particles and seed pods. You have done it again, you've kept me sane. Thanks for that.