Stratigraphy and paleoecology at the Wenlock/Ludlow boundary on Saaremaa Island, Estonia — An abstract submitted to the Geological Society of America for the 2011 annual meeting

July 7th, 2011

KURESSAARE, ESTONIA–Editor’s note: The Wooster Geologists in Estonia found enough material, and had enough time, to write abstracts for posters at the Geological Society of America Annual Meeting in Minneapolis this October. The following is from student guest blogger Nick Fedorchuk in the format required for GSA abstracts:

STRATIGRAPHY AND PALEOECOLOGY AT THE WENLOCK/LUDLOW BOUNDARY ON SAAREMAA ISLAND, ESTONIA

FEDORCHUK, Nicholas D., WILSON, Mark A., MATT, Rachel M., Dept of Geology, The College of Wooster, 944 College Mall, Wooster, OH 44691-2363, VINN, Olev, Dept of Geology, University of Tartu, Ravila 14A, 50411 Tartu, Estonia

The boundary between the Wenlock Series and the Ludlow Series can be easily observed on the island of Saaremaa in western Estonia. Here, the boundary is distinguished by a major disconformity that can be correlated to a regional regression described in several previous studies. During this time, western Saaremaa was a lagoonal facies that reflected sea-level changes within the Baltic Basin. We measured and described this Wenlock-Ludlow boundary interval at Soeginina Cliff on the western shore of Saaremaa. Here this boundary consists of the Vesiku Beds of the Rootsiküla Formation (Wenlock) overlain by the Soeginina Beds of the Paadla Formation (Ludlow). The Vesiku Beds (Wenlock) record a carbonate lagoonal environment with finely laminated beds and Thalassinoides burrows (indicating oxygenated bottom conditions). The fauna is much less diverse than that in normal marine sediments of the Wenlock. The top surface of these beds (the primary discontinuity surface) shows a microtopography and dissolution consistent with exposure and abrasion. The top 20 centimeters also show diagenetic alteration of the laminated sediments, probably from fluids traveling through the Thalassinoides burrow systems. The Soeginina Beds (Ludlow) show pulsating transgressive sediments with multiple discontinuity surfaces. Large oncoids are common in these beds. They have distinctive shapes because they were initially spherical and later stabilized and grew like small stromatolites upwards. These forms may indicate periodic energy reductions in these transgressive waters. There are also storm beds with biogenic debris including oncoids nucleated on gastropods. This boundary interval is topped by thin dolomites and stromatolites. This example of the Wenlock-Ludlow boundary can be correlated with other such disconformities recorded in a variety of depositional environments, such as in the equivalent reef complexes of Gotland, Sweden.

Wooster Geologists return to Suuriku Cliff, Saaremaa, Estonia

July 7th, 2011

KURESSAARE, ESTONIA–Today we visited one of Rob McConnell’s (’10)  Senior Independent Study field sites on the northwest coast of Saaremaa. Suuriku Cliff (N58.50875°, E21.99818°; see above image) is an exposure of the Jaani Formation (Lower Silurian, Wenlock). There are two members here: the upper Ninase (most of the cliff) and the lower Mustjala. Rob sorted out the paleoecology and environments of deposition of these two members using samples from this location and two others.

We were here today to find additional crinoid calices to continue a project Bill Ausich, Olev Vinn and I are pursuing. We found a few, too, although none very photogenic. It was also a chance for us to see more examples of Silurian limestone and fossils before we leave the island on Saturday.

Nick Fedorchuk and Rachel Matt at Suuriku Cliff. We want to show that some Saaremaa cliffs really are more than a meter high!

Panorama of Tagalaht Bay south of Suuriku Cliff near Veere. This bay is where German naval and infantry forces invaded Saaremaa on October 11, 1917, in Operation Albion.

Return to a Silurian crinoid forest

July 5th, 2011

KURESSAARE, ESTONIA–Today our Wooster Geology team visited a favorite outcrop of mine: the Äigu Beds of the Kaugatuma Formation exposed on the northwestern shore of the Sõrve Peninsula on Saaremaa. These are Late Silurian (Pridoli) limestones with a great abundance of crinoid fragments — so many that they are locally called “ringstones” (see the above image of a crinoid stem and isolated ring-like columnals). Palmer Shonk (’10) studied this outcrop for his Independent Study thesis. We returned here today to collect more field data so that Palmer, Olev Vinn, Bill Ausich and I can write a professional paper on the depositional system and paleoecology.

The limestones show significant storm beds made of skeletal debris, especially the crinoids but also corals, stromatoporoids, bryozoans, brachiopods and trilobites. The most remarkable aspect of this exposure is the presence of an in situ crinoid “forest” of holdfasts (the part of the crinoid that holds it to the sediment). Our job today was to find out more about the non-crinoid fauna since Palmer and I have already mapped out the crinoid holdfasts. We hope to soon publish a rare look at a Silurian crinoid community essentially preserved in place.

The Kaugatuma outcrop on the Sõrve Peninsula of Saaremaa. This was a Russian amphibious landing zone in 1944. You can bet you’ll read more about that story later!

The star-shaped fossil with the hole in the center is a crinoid holdfast in place at Kaugatuma. There are dozens of these scattered across the outcrop. The crinoid is Enallocrinus. The hole in the center is the hollow lumen of the crinoid stem.

Nick found this calyx of another crinoid known as Eucalyptocrinites. Such well preserved calices are rare.

Nick also found this very nice trilobite pygidium (tail piece). It is preserved well enough that we can probably later identify it to the genus level.

Back to the Big Island for the Fourth of July

July 4th, 2011

KURESSAARE, SAAREMAA, ESTONIA–The Wooster Geologists Estonia Team today braved the Baltic Sea again and took a ferry from the island of Hiiumaa to return to their previous field sites on the island of Saaremaa. We worked at Soeginina Cliff on the western coast (shown above) to finish up Nick Fedorchuk’s Independent Study field research on the Wenlock-Ludlow (Silurian) erosional sequence. It was a fun day because we now have hypotheses to test about these rocks and fossils. More on those later.

The above rock shows an advantage we have studying exposures on rocky coastlines. The waves erode blocks of limestone from the cliff and polish them up on the boulder-strewn beach. We can thus see our rocks in three dimensions rather than just the flat cross-section we would normally have. The trick, though, is to know from where in the section the boulders were derived!

We found some excellent sections through the numerous oncoids in our outcrops as well. We saw that many are formed around gastropod (snail) shells — very much like a Jurassic version I recently described as one of Wooster’s Fossils of the Week.

Tonight we will celebrate the 4th of July with a pizza dinner in downtown Kuressaare. It is as close to American food as we can get here. On this holiday we salute the Wooster Geologists Iceland Team also far from home on the front lines of science!

Saying goodbye to the little island of Hiiumaa

July 3rd, 2011

KÄINA, ESTONIA–Today we had our last visit to our Silurian quarry working site (where I photographed the Paleofavosites coral fossil above, which by the way was preserved upside-down in the sequence), and then we had lunch in the town of Kärdla overlooking the Baltic. Tomorrow we take the early morning ferry back to the larger island of Saaremaa where we resume fieldwork. Here are a few last photographs from Hiiumaa.

The other Silurian outcrop on the island: Kallaste Cliff. A bit overgrown, we think.

Some purple flowers found in the woods near our field site.

Yellow flowers in the quarry itself. I do know the one on the left is a daisy!

Whitish flowers and then a moth-covered thistle. I photographed this Six-Spot Burnet moth earlier, but three on one flower deserved another image. I'm sparing you the photos of them mating!

Our hotel on Hiiumaa. For most nights we were the only ones there. The students said it reminded them of "The Shining".

A walk to the sea after lunch in Kärdla. We have enjoyed this weather very much.

Quarry time on Hiiumaa

July 1st, 2011

KÄINA, ESTONIA–Rachel, Nick and I worked today in our lonely quarry on Hiiumaa measuring and describing this section of Lower Silurian (Llandovery, Rhuddanian) rocks and fossils. This is the fieldwork for Rachel’s Senior Independent Study.

One of the dilemmas is the nature of the lower interbedded limestones and shales. In places they show gently sloping beds and curved tops as here. Does this indicate some sort of mud mound or bioherm? Or is it a function of slumping in the quarry itself? (I'm leaning toward the latter.)

The fossils here are excellent, including corals and bryozoans. (Just because I could I expanded the image of the mite!)

A second new Senior Independent Study project begins in Estonia

June 30th, 2011

KÄINA, ESTONIA–Today we moved our geological investigations from Saaremaa to the island to the north: Hiiumaa. Our friend Olev Vinn of Tartu University then led us to an abandoned quarry in the Hilliste Formation (Lower Silurian). This made Rachel Matt very happy as it is the place she has been studying as the potential site for her Senior Independent Study project. First, it is not filled with water; second, it is easy to get to; third and most important, it has a diverse mix of fossils and rocks crying out for her attention. She is shown above in front of what looks like an ancient mud mound.

The rocks and fossils are so good that the students quickly accumulated a pile of cool specimens. I had to stop them from picking up fossils because we’ve not even started to sort out the stratigraphy. A good sign!

Our friend Bill Ausich at Ohio State University will be pleased to hear that there is much crinoid debris, as shown here in this image. It is a calyx plate in the center, with stem fragments around it. Hiiumaa is the island on the Euro to the right of the top star.

There is much I can’t identify here, at least not immediately. Another good sign!

The only thing that I don’t like is that we’re on yet another island, and this one smaller than the last. So much water around so little land, and no way off except by these little ferries. Our car is the brown one in the front on the right. I’ll endure for science!

Saaremaa Silurian stromatolites studied

June 29th, 2011

KURESSAARE, ESTONIA–Our fieldwork today at the Soeginina Cliff locality ended with an examination of a sequence of stromatolites near the top of the exposed Ludlow section. Stromatolites are layers of sediment accumulated by photosynthetic cyanobacteria. They are the earliest fossils known, some 3.5 billion years old, and these structures are still being formed today. Bacteria were present at the beginning and no doubt will be the only surviving life at the end.

In the image above, the stromatolitic portion of the outcrop begins at Nick’s upraised arm and goes almost to the top of the exposure. It is a complicated story because they seem to be sitting on an erosional surface cut into the dolomite underneath. There are also patches of what appears to be gravel under some of the stromatolite domes. A dolomitic sand fills the spaces between the stromatolite heads. Stromatolites can tell us a lot about the paleoenvironment of this area during the Silurian.

Closer view of the stromatolites at Soeginina Cliff.

Top view of the Soeginina stromatolites showing the fine layering produced by cyanobacteria. (Note the clever use for scale of a Euro with the map of Estonia on it. You can easily pick out the island of Saaremaa!)

I don’t usually come across stromatolites in my work. The last time I saw a few was with Matthew James on a great field trip to British Columbia. Part of the joy of supervising student research is that I must learn alongside them!

Independent Study fieldwork begins in Estonia … with a little unexpected canine companionship

June 28th, 2011

KURESSAARE, ESTONIA–Nick Fedorchuk began his fieldwork today at the Soeginina Cliff site we visited two days ago. The first thing we did was scout out the best place to measure the most complete section possible, and then we started the slow process of sampling and describing the rocks and fossils. On average we did about a meter an hour.

The above image shows one of the curious oncoids in the Soeginina limestones. Oncoids are usually almost spherical because they rolled around as bacteria formed layers around a nucleus. The oncoids in the lowermost Ludlow (Paadla Stage) here show an initial formation as spheroids and then they sat still on the seafloor and grew upwards to make little layered caps. The oncoid was knocked over occasionally and a new cap grew on top of the sideways oncoid. This finally made oncoids with multiple growth directions visible in cross-section.

Above is a bedding plane view of an oncoid-rich layer with shelly fossils. Some of the oncoids have formed around gastropod shells.

The trace fossils (evidence of organism behavior) are especially interesting because we can see them in bedding plane view (as above) and also in cross-sections. We will look at their distribution using various ichnofabric indices.

At the start of our day on the outcrop this happy Estonian dog joined the party. It stayed with us the whole time. It liked to splash around in the ocean and then joyfully jump on us — not conducive for taking notes or whacking rock samples, but fun nevertheless. Wolf (maybe the name we gave him was too easy) loves to gnaw on the carcasses of large, long-dead seabirds, bringing them to us as we worked. Wolf was sometimes a bit too exuberant, but he was a good friend for the day. We hope to see him tomorrow at the same place!

An intricate Silurian stromatoporoid reef on the island of Saaremaa, Estonia

June 27th, 2011

KURESSAARE, ESTONIA–Stromatoporoids are extinct calcareous sponges that were very common in shallow water environments of the Silurian. They are especially abundant in the middle Silurian of the Baltic Region. Today we visited a site called Katri Cliff where a reef composed of stromatoporoids is exposed. Olev Vinn is shown above studying them (with the inevitable remains of a Soviet coastal border guard post in the background).

Stromatoporoids made hard, dense skeletons of calcite, sequentially adding layers to them like onions. At Katri Cliff we found many examples of these sponges with rugose corals and tabulate corals embedded inside them. Apparently the sponge grew up around the coral skeletons, immuring them alive. The interesting question is whether the sponges and the corals had a mutual beneficial relationship or if they were actually competing for resources like space and food.
Stromatoporoid showing conical rugose corals in its skeleton.
Stromatoporoid broken in half and revealing an embedded tabulate coral.

We have placed this ancient reef on the list of possible projects for Rachel, but we won’t know what she is going to pursue until we visit the nearby island of Hiiumaa at the end of the week.

And in case you’re tired of so many fossils and seascapes in this blog, here’s another bit of history we saw today: Below are trenches built at the top of Ninase Cliff. The tragedy of 20th Century Estonian history is that we can’t immediately tell who dug these trenches. Was it Imperial Russians in 1917 defending against the invasion of Imperial Germans? Could they have been built by Soviets against the invading Nazis in 1941? Or maybe Nazis in 1944 fighting the re-invading Soviets? There is some satisfaction on this part of the coast to observe that the sea is slowly eroding these trenches back into the ancient limestone gravel from which they briefly appeared.

« Prev - Next »