Monday, May 28, 2012

Day 6 - Hudson Valley Fold-Thrust Belt

Yes, I got lazy and didn't post after every outing of the Geology of the Hudson Valley field course at the end of each day.  I was just too damn tired in the evenings. It's not like I get paid for this blogging shit.  Because of the lousy intermittent rain, I stopped bringing my camera but, fortunately, some of my students brought theirs and I'll steal their pictures.

Anyway, on Wednesday, May 23, we examined early Devonian rocks from the Helderburg Group up through the Hamilton Group (for those who know what I'm talking about) exposed in the mid-Hudson Valley from Kingston up through Catskill.  These rocks consist of a whole series of limestones above the Taconic angular unconformity and extending upward into more shaly rocks.  They're all marine witnessed by the abundant fossils of marine invertebrates including numerous species of bryozoa, brachiopods, gastropods (snails), trilobites, and corals (all of which we found in these rocks).

The Taconic angular unconformity.  One day I'll write an entire post on
what this represents but for now I'll just say it shows the tilted Late Silurian
Rondout Formation on the left atop the almost-vertical Mid-Ordovician
Austin Glen Group on the right.  Route 23, Catskill.

Even more significantly, these rocks are heavily folded into upward arching anticlines, downward sagging synclines, and cut through with fractures (joints), thrust faults, and mineral-filled veins.  This package of early Devonian rocks in the Hudson Valley have been significantly shortened in a roughly east-west direction.

Hiking on vertically-bedded fossiliferous Becraft Limestone on the Catskill Creek.

These rocks form a small-scale fold-thrust belt (it has folds and thrust faults) that is very well-exposed and easy to study.  Still, generations of geologists haven't really settled one important fact about these rocks - which orogenic event(s) deformed them - Acadian (Devonian) and Alleghanian (Pennsylvanian/Permian) or two different phases of the Alleghanian.

1 comment:

  1. My Structural class poked around the Catskills, and we stopped by that awesome angular unconformity. It was a cold day, and one girl kept getting wonky strike measurements. After a few minutes of puzzling, someone noticed she was wearing mittens with little magnetic clasps! It was a good lesson in choosing appropriate field gear.
    Anyway, I've only got about three sentences worth of information on that outcrop, so I'll be looking forward to your in-depth post!