Tools of My Trade (At Least for Today)

September 3rd, 2009

tools090309

MAKHTESH GADOL, ISRAEL–The fieldwork could not have been better, although if you watched me all day in the desert sunlight you would have thought otherwise. After I hiked up into the Matmor Hills to find the right horizon, I spent hours in the same place collecting fossils off the surface and sieving the sediments to obtain tiny shells (especially of thecideide brachiopods). The goal is to thoroughly understand the paleontology of this unit, including how these organisms interacted with each other in that ancient Jurassic sea. The persistence paid off with a diverse set of brachiopods, corals, sponges, echinoids, serpulid worms, bivalves, gastropods, and the first ammonite I’ve seen in the Matmor Formation. There is enough complexity in this one site to support at least another two Senior Independent Study projects.

This view of a coral in cross-section shows how complex bioerosion can be.  You can see several holes in the brown coral matrix filled with white sediment.  Inside these borings are cross-sections of bivalve shells.  Note that some borings have more than one set, meaning the hole was occupied by nestling clams after the borer died.  The patches of shiny grey are silicified regions of the coral skeleton.  Since the coral was aragonitic, its original skeleton has been replaced by several minerals.

This view of a coral in cross-section shows how complex bioerosion can be. You can see several holes in the brown coral matrix filled with light tan sediment. Inside these borings are cross-sections of bivalve shells. Note that some borings have more than one set, meaning the hole was occupied by nestling clams after the borer died. The patches of shiny grey are silicified regions of the coral skeleton. Since the coral was aragonitic, its original skeleton has been replaced by several minerals.

Goal!

June 8th, 2009

BABINO, LENINGRAD REGION, RUSSIA–Today we visited an active quarry, which is a different experience from the riverbank exposures and abandoned quarries we have been frequenting.  Quarry mud has a special character — a kind of purified mud, the kind of mud all mud aspires to be.  There are also very large trucks splashing by, giant rock saws whining, cranes lifting large blocks, and small groups of curious workmen who want to see what we are doing there with our hammers that now seem so small.  Active quarries can produce the very best exposures for geologists, especially those interested in the boundaries between rock units as we are.  This quarry at Babino N60.03035°, E32.38613°) is particularly good because they quarry Ordovician limestone by first cutting it vertically, and then lifting the rocks away in sections, revealing smooth surfaces perpendicular to bedding.

Cut surface through Ordovician section, Babino Quarry.

Cut surface through Ordovician section, Babino Quarry.

I want most to see the boundary between the Lower and Middle Ordovician rocks, and look at the trace fossils above and below it.  This boundary — a plane in the rocks which extends across northeastern Russia, Scandinavia, and parts of northern Europe — could not be better displayed than the way we saw it here.  It is an erosional surface which has been cemented into a carbonate hardground and then bored (to some extent that we are debating) and abraded smooth.  Above it is a significant change in the fossil fauna, a change which can be seen around the world.  In no place is this boundary better presented to geologists than here.

Lower/Middle Ordovician boundary in the Babino Quarry.

Lower/Middle Ordovician boundary in the Babino Quarry.

The trace fossils along this boundary are complex and may show both boring and burrowing behavior.  The distinction depends on when the sediments were soft, firm and cemented, and on the varieties of organisms which did the work.

Borings in the Lower/Middle Ordovician boundary at Babino Quarry.

Borings in the Lower/Middle Ordovician boundary at Babino Quarry.

I can’t take these specimens home for further examination.  I’d very much like to make thin-sections (slices of rock shaved down until almost transparent for microscopic analysis) of all the critical intersections, but that will have to wait.  Andrey collected many samples he can cut up and share from his lab in Moscow.

Ordovician Hardgrounds

June 7th, 2009

SYAS RIVER, LENINGRAD REGION, RUSSIA–The main geological attractions for me on this expedition are the abundant carbonate hardgrounds in the Lower and Middle Ordovician in this part of the world.  A carbonate hardground is a cemented seafloor.  What were soft sediments on the bottom were cemented with carbonate minerals (calcite in the Ordovician) so that they became a rocky surface several centimeters thick.  The sediment is usually carbonate mud and shells, so the result is essentially a limestone seafloor.  Many invertebrate animals colonize these hard surfaces by wither encrusting them or boring into them.  Those eocrinoids illustrated earlier, for example, often encrusted Early and Middle Ordovician hardgrounds.

Today at the Syas River Carbonate Mound locality (N60.02316°, E32.62471°) we saw numerous hardgrounds bored by a shallow variety of a trace fossil called Trypanites.

Borings in Ordovician hardground fragments.

Borings in Ordovician hardground fragments.

These are the most common borings in hardgrounds.  This particular type of Trypanites is remarkably shallow — often appearing as pits rather than the usual penetrating cylinder.  Another difference between these hardground fossil faunas and those I know best in North America and western Europe.

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