CTgeo
A TRIP INTO THE HARD ROCK TERRANES OF SOUTHEASTERN CONNECTICUT

We will visit several famous roadcut exposures of rocks of several terranes in eastern Connecticut, a pegmatite quarry, and an abandoned mine. Please be sure to stay off the highway pavements, and watch out for falling rocks! Permission from the state highway dept. is needed to stop on Rte 9, but we will get off at intersections and walk back. Rock collecting in state forests at stops 3 and 9 are not allowed without a permit. Apply in writing to Dr. David Leff, Assistant Commissioner, Ct. DEP, 79 Elm St., Hartford, CT 06106.

Unlike most of our terranes, the Hartford Basin terrane is formed "in place" as a low area that collected late Triassic through Early Jurassic sediments from from 225 m.y. to 175 m.y. ago.  The Bronson Hills terrane is believed to be partly the roots of an ancient 440 m.y. "volcanic island arc" in the Iapetus Ocean, before the arc collided with the ancient continent. Its Ordovician rocks were re-metamorphosed during the Acadian orogeny about 380 m.y. ago. The Bone Mill fault forms a a border with the Merrimack terrane to the east (named after its exposures along the Merrimack River of NH). This terrane shows both Acadian and Avalonian metamorphism of Ordovician rocks, with pegmatites melting from the crust during the Avalonian heating event. To the south and east, the Honey Hill fault and other faults mark the border with the Avalon terrane, which has late Precambrian gneisses (about 600 m.y. old) that escaped Acadian metamorphism but instead only show metamorphism from the Avalonian orogeny of about 175 m.y. ago. The Avalon terrane in particular has been proposed to have moved very long distances along and into the ancient continent to the west.

TERRANES.JPG (140950 bytes)

A map of terranes in eastern Connecticut by Robert Wintsch, including dates of final formation of rock units.

From Wesleyan, head south on Rte 9. The highway rises up the great eastern border fault of the Mesozoic Hartford basin, into early Paleozoic rocks (mainly gneisses) of the Bronson Hills terrane, which runs up the eastern side of the Connecticut River Valley into New Hampshire. It is believed that the border fault is formed along the western terrane border of the Bronson Hills; in other words, it is an early Paleozoic crustal "weak" structure along which the Triassic and Jurassic fault activity occurred.

Stop 1. Exit 9, park along Rte. 81 and walk back. The Higganum dike is well exposed here. We will examine the columnar cooling fractures and other features of this great basalt dike, which has been dated as 201 m.y. old.  Along Rte. 9 nearby to the south, the Ordovician Monson gneiss of the Bronson Hill terrane shows "s" folds (used on the cover of "The Face of Connecticut").  Here is an old photo:

Ponset.jpg (69673 bytes)

The Higganum dike at Exit 9, Route 9 is the root of a huge volcanic fissure eruption that produced a flood basalt 200 m.y. ago.

Stop2. Exit 8, Rte 9. Along a parallel road, we will examine a pegmatitic gneiss of the Ordovician Middletown formation that contains cordierite and other interesting minerals.

Stop 3. Beaver Meadow Road, Cockaponset State Forest. Behind a state garage (an old CCC camp), take the trail south along the ridge to an abandoned pegmatite quarry. There is good muscovite and other minerals to be seen here. Many pegmatites of the Middletown district were mined, the last one closing only a few years ago. These pegmatites are much younger than the gneiss around them, melting from the crust about 275 m.y. ago.  The large black crystals in this photo are tourmalines.

Pegmatite.jpg (134258 bytes)

 

An abandoned pegmatite quarry in Haddam contains large crystals of mica and tourmaline.

Stop 4. Exit 3, Rte 9, near Essex, park near the north entrance ramp. We are close to the contact with the Avalon terrane, which on the other side of the river is the Honey Hill fault. The Ordovician Tatnic Hill formation in the Merrimack terrane is a gray gneiss squeezed up against the Late Precambrian (c. 600 m.y.) Rope Ferry gneiss, which is not far down the road to the east.

Stop 5. This is a future roadcut for Rte 11, whenever it gets finished. The pink Hope Valley alaskite gneiss (late Precambrian) is spread in great folds through SE Ct and southern RI in the Avalon terrane.

fault.jpg (55825 bytes)

 

Faults shows layers that do not match up across the break.

Stop 6. Rte 11 roadcut. This fault in the following photo is believed to be Triassic or Jurassic in age, related to the same events that created the eastern border fault near Middletown (gneisses of the Avalon terrane).

 

 

boudins.jpg (60520 bytes)

 

The Hebron gneiss is also well exposed below Gillette's Castle.

Stop 7. Rte 11 roadcut . This is the Ordovician Hebron gneiss in the following photo, a major rock type of the Merrimack terrane.  Notice how a pegmatite layer in the gneiss has been squeezed into "sausage links," a feature called boudinage, which is common throughout the Hebron gneiss. Also examine the grooves made during drilling to blast out the roadcut. Some grooves have been faulted, demonstrating that tectonic activity continues.

 

Brimfield.jpg (74615 bytes)

Acidic runoff from Brimfield roadcuts can pollute the local streams.

Stop 8. Rte 11 roadcut.  The photo shows Ordovician Brimfield schist, which structurally lies beneath the Hebron and is exposed as the inside of one of the "domes" of the Merrimack terrane. This formation is infamous for its iron sulfide minerals, which rust and corrode upon exposure to the air. Some ditches along the highway contain very acidic runoff from rain due to the chemical action of the sulfur. Some people who have water wells in the Brimfield have to have their water treated to remove the iron and sulfur.

Cobalt.jpg (58547 bytes)

The Cobalt mine tunnels should not be entered.

Stop 9. Cobalt, in the Meshomasic State Forest. The old cobalt mine is in the Ordovician Collins Hill schist, a formation in the Bronson Hills terrane. Do not approach the pits nor enter any tunnels such as you see below! The schist has veins of cobalt sulfide minerals, which were mined in the 18th and 19th centuries to be used for cobalt blue glass. There are quartz veins with shiny white arsenopyrite (AsFeS2) nearby, and at an old tunnel by Mine Brook you can pick out pretty violet colored garnets (most are small). Nearby is a quartz vein that contains native gold, on private property (unfortunately for field trippers). It has been prospected recently but no mine is seriously proposed.  You can read about Cobalt's mining history at Mysterious Goblins of Cobalt.