The discovery of dinosaur eggs from Cretaceous fluvial sediments near the village of Ariyalur in the state of Tamil Nadu , South India is getting lots of press cover ...here and here. Hundreds have been found in clusters of about 8 over an area of about 2 sq km. Looks likely to be a nesting site. On a sadder note I read in the Times of India a few days ago that there has been no protection given to the site by the government despite requests from the scientists. Locals are already taking away the fossils eggs and disturbing and damaging the site in the process. What a shame!
There also has been some silly press coverage calling this a Jurassic treasure trove, a holdover of the popular link between anything dinosaur and the word Jurassic....as in Jurassic Park the movie. This particular south Indian sedimentary basin does not have Jurassic sediments. It contains an Early Cretaceous to Early Cenozoic sequence.
What caught my eye was that the sedimentary layer containing the eggs were capped by a volcanic layer. The scientists from Periyar University, Salem, seem to think that this volcanism represents the Deccan volcanic activity dated to around 65 million years ago and that the field relationship between the volcanic layer and the underlying sediment could suggest that this particular volcanic event may have killed or damaged those eggs.
I don't know enough details about the deposit to answer that with certainty but I did have some random thoughts on event deposits i.e layers deposited almost instantaneously and how geologists use such deposits to ascertain the age of the associated sediments. In this case they suggest that sedimentation and the volcanism took place in quick succession and by quick I mean volcanism took place immediately after the dinosaurs laid those eggs....before those eggs hatched.
If a sediment unit is capped by a volcanic layer that is dated to say 65 ma (million years old) then that would mean that the sediment cannot be younger than 65 ma. But does the relationship mean that the sediment too is 65 ma? And what does it mean when you say the sediment is 65 ma. How much 65 ma plus minus ...years, or uncertainty is there in a calculation like this?
Lets say we get lucky and that the sediment layer is sandwiched between two volcanic layers that can be dated with radiometric methods. Let's say both layers give a date of 65 ma. What does that tell you? Radiometric dates that old come with a sizable uncertainty on the order of hundred thousand years or so. That means the date of the overlying layer might come out as say 65 ma with an uncertainty of 300 thousand years. The date of the layer underlying the sediment may come out say 65.2 ma with an uncertainty of 300 thousand years. The two dates are statistically unresolvable. So even if the sediment is sandwiched between two volcanic layers that indicate the same statistical date there could still be a time lag of tens to a hundred thousand years or so between the sediment being deposited and the volcanic activity.
Fossils are not much use either for this purpose. Fossils can eliminate the possibility that the events took place in quick succession if the sediment contains fossils which are obviously much older that 65 ma. But again fossil species have temporal ranges of a hundred thousand years or more. Even if the sediment underlying the volcanic layer contains fossils that are indicative in this case of the latest Maastrichtian age (close to 65 ma) there will be an uncertainly of tens of thousands of years and so their presence won't resolve events taking place on smaller time scales.
Geologists have then to rely on the detailed physical relationship within and between the sediment and the volcanic material.
Preserved sedimentary layers are mostly time averaged deposits. That means that material at the bottom of a particular bed is not necessarily older than the material in the upper part of the same layer. During deposition waves and currents keep reworking the same bundle of sediment. Animals may burrow into it and churn up sediment. Material at the bottom part may get transported to the upper part of the layer. The layers thus becomes time-averaged. Organisms who have lived at different times through that depositional episode are all distributed randomly - with respect to their age - throughout the deposit.
A good recent example of a time averaged deposit is the sediment unit that contains the remains of Ardipithecus ramidus the early hominin found in Ethiopia. Scientists lucked out there and realized that the sediment is sandwiched between two volcanic layers dated to about 4.4 ma. Each layer differed in their radiometric age by about 30 thousand years with an uncertainty of about 75 thousand years. Based on this information and the internal characters of the layers the scientists concluded that the geological unit was a time averaged deposit representing a few thousand years of deposition.
This layer containing the dinosaur eggs does not have the characters of a time averaged deposit at least not one representing hundreds or thousands of years. The eggs were found in clusters of 7-8 eggs per nest and these nests are found in successive layers of the sequence. These two indicators suggest that the sediment was not disturbed much. Rather the setting, fluvial floodplains would have meant that the eggs were buried rapidly and entombed during seasonal floods, sorts of event deposits just like volcanic eruptions. That may have been the main cause of the eggs not hatching. The layers immediately underlying the volcanic cap may thus represent sedimentation taking place over a few tens of years, each nest bearing layer essentially preserving or freezing ecosystem conditions as they existed at that time. There is the possibility that the uppermost layer containing eggs represents maybe the last egg laying season before the eruption.
Volcanism when it occurred would have sealed the deposit from further damage from the elements. How close in time was that event to the last egg laying season? Again the scientists will have to look closely at the relationships. Are the eggs in the uppermost layers caked with volcanic ash? Do they show signs of being baked or cracked due to heat, a kind of a Cretaceous hard boiled feast?
Detailed sedimentology and stratigraphy will provide more clarity than absolute radiometric dating and fossil ranges when posed with questions of this nature.