Monday, November 25, 2013

Chazy Reef - Part I

So after visiting Ausable Chasm this past October, and spending the night in Plattsburgh, we drove into Vermont at Rouses Point, a stone's throw from Canada, and then followed Route 2 down through the islands in northern Lake Champlain toward Burlington, VT.

Cropped from Google Maps

Along the way, however, we made a detour to Isle La Motte (island just east of Chazy and the NY/VT line above) - site of the geologically famous Chazy Reef.  Never heard of it, you say?  It's unfortunate because it's one of the coolest fossil localities in the state.  And it's just sitting there - free and open for anyone to wander in and have a look.

If you go back a read my previous post about Ausable Chasm, you'll learn about the Potsdam Sandstone.  Starting half a billion years ago, when New York State was a flat featureless landscape of rock and gravel (no life yet on land), the sea began to encroach as sea levels gradually rose.  It took a few tens of millions of years, but eventually the whole of the state was covered with warm shallow seawater (we were, believe it or not, closer to the equator and in the Southern Hemisphere to boot).  The sandy seafloor (later to become the Potsdam Sandstone exposed so nicely in Ausable Chasm) gave way, after a time, to a seafloor of carbonate or lime mud - grains of the mineral calcite (CaCO3) which eventually formed limestone.  The Chazy Sea was established.

Carbonate mud seafloor in the Caribbean - just like the ancient Chazy Sea

The details are far more complex, but that summary will serve well enough for our purposes.

So here's a paleogeographic map depicting a part of the Earth some 480 million years ago during a span of time geologists call the Ordovician Period.  Look carefully at this image.  See the equator (Eq) line near the top?  See the 30° S latitude line near the bottom?  In between is proto-North America, called Laurentia by geologists, rotated clockwise by 90°.  The modern-day states are outlined, can you identify the sideways New York near the lower right?  See that red dot?  That's the location of the Chazy Reef as it was forming.

Ron Blakey, Colorado Plateau Geosystems, Arizona USA.

Note that the Chazy Reef was around 30° S latitude just offshore in a shallow, subtropical sea.  That chain of islands to the south are a line of volcanoes ominously approaching North America, eventually to collide, but nothing to worry about now.  It's all sunshine and gentle trade winds in the Chazy Sea.  A beautiful place to live if you were a marine invertebrate.

And marine invertebrates did live there, building a reef system over time that rivaled the Great Barrier Reef in our modern world.  The Chazy Reef stretched for 1,000 miles (1,600 km) from Quebec to Tennessee.  Only small patches of fossil material remain of this once massive reef and one such place to preserve it is Isle La Motte in Lake Champlain.

The Great Barrier Reef - what upstate NY looked like 480 million years ago

There are two sites on the island designated as U.S. National Landmarks and protected by the Isle La Motte Preservation Trust - the Goodsell Ridge Preserve and the Fisk Quarry Preserve.  In my next post, in a day or two, I'll show some pictures of fossils from these amazing sites.  For now, I'll leave you with this:

Limestone bedrock on Isle La Motte.  The spiral is the cross-section of a fossil gastropod (marine snail) named Maclurites magnus.  Just laying there for all to look at.  Cool, isn't it.

More to come...

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