Colby Geology: Triassic-Jurassic Hartford Basin Field Trip, Connecticut

A Colby Field Trip to the
Hartford Basin of Connecticut

Colby College
Waterville, Maine

Click on any photo for an enlarged view!


The Hartford Basin of Connecticut is one of a number of rift basins that began opening along the modern East Coast of North America as the Atlantic Ocean was struggling to open during the late Triassic and early Jurassic Periods, some 180 million years ago. Colby students have come here in the spring terms of 1993, 1996 and 1998 to study the geologic history of these events, a geologic history repeated in numerous similar basins that together contain rocks geologists refer to as the Newark Supergroup. The trip is taken in conjunction with the historical geology course (GE142), but is open to any student who has taken or is currently enrolled in physical geology (GE141).
The costs of transportation and lodging for the trip are covered by special endowed funds in the Department of Geology, so students' out-of-pocket expenses are limited to their own food and incidentals. Thus, any student who wants to go on the trip is free to do so.
On the margins of the Hartford Basin, older basement rocks, such as the fine-grained gneiss being examined above (right) by Jay Zarnetske '00, are exposed. In places, pegmatite dikes of younger granitic rocks, dominated by quartz, orthoclase feldspar, and black tourmaline, can also be seen, as shown in the photo at the above left. [Photo courtesy of Jay P. Zarnetske.] It was these rocks that were weathered and eroded to provide the sediments that were ultimately deposited in the basin. Quartz and feldspars, and some of the micas and other heavy minerals, survived as clasts in the new rocks. Some of the less resistant minerals weathered to produce the clays and hematite that stained the sediments bright red and cemented them together to create the sedimentary rocks shown a little later on this page.
The rifting process was created by the upwelling of magmas from the Earth's interior, which in places erupted as basalts that are interbedded with the lake deposits. Above, Colby students study an overview of the Higganum Dike, one of the main feeder units through which magma made its way to the Earth's surface, prior to going down to actually look at the rocks themselves. (The roadcut shown at the top of the page cuts across the dike at this site and is the same one visible on the other side of the students in this photo.) The magma that erupted through the Higganum Dike became the Talcott Basalt, a pillowed basalt unit at the base of the Jurassic sequence in the valley. The pillows apparently resulted from subaqueous eruptions in a lake formed by damming of the streams (which were depositing the New Haven Arkose) by the basalt flow itself.
In the photo to the left, Colby students are checking out this contact between the basalt and the underlying New Haven Arkose along Route 4 in Farmington. Some of the first coins minted in the American colonies were produced from copper mined along this contact - copper in concentrations that are far from economic in today's market.
The rifting process also created an enclosed lake basin, which became the site of deposition of fine-grained sediments. These silts and clays slowly settled to the bottom to become mudstones and shales. Alternating colors from red to gray and black represent fluctuating lake water depths and availability of free oxygen in the ancient lake waters. Red shales mark periods of playa lake formation in the desert-like climate of the time, and darker rocks, in general, were deposited in deeper waters of more permanent lakes. Mudcracks and rare raindrop impressions also attest to fluctuating water levels and occasional drying of these lake beds, and the black shales yield rare but excellently preserved fossils, particularly of various plant remains, and occasional fish.
Muscovite (mica) flakes deposited on the bedding planes in the shales and sandstones make it possible for individual surfaces to be readily distinguished, which is beautifully demonstrated by the units that are exposed at Dinosaur State Park in Rocky Hill. Rocks here display hundreds of early Jurassic dinosaur footprints in one of the largest such concentrations known in North America.
Lots of neat things (a technical term for interesting specimens!) can be found in the scree at the foot of the slopes, from pieces that have fallen off in the winter freeze-thaw cycles, and this is often a rewarding area to search, as Colby students have found. Mudcracks abound in some of the units, and some even show pyritization - recrystallization of iron sulfide - that shows that water flow in the subsurface was probably concentrated alone the margins of these ancient sedimentary contacts. The gray and black shales also sometimes have numerous minute crystals of pyrite in perfect silvery-yellow cubes. Cross-bedding and other informative sedimentary structures are also common in the units, particularly those that are more coarse-grained.
Jay Zarnetske '00 and Debbie Schwartz '01 were among those who struck out on their own to search the cliffs for specimens, shown in the photo above right. Less material can be seen this way, but one DOES get a chance to be the very first to see anything, and it is a good way to examine up close the stratigraphic relationships. One just has to be careful not to be taking home a badly skinned knee - or worse - as a memento of the trip!
Intercalated within the sedimentary sequences are two more basaltic units, one of which, the Hampden Basalt, is readily accessible in the area of Cromwell. Above, Kevin James '00, Debbie Schwartz '01, and Anna Randall '01 are checking out some of the rubble that was exposed at a construction site. In the photo to the right, Kevin, Michael Eller '00, and Anna are looking at some of the zeolites found in vesicles and vugs in the basalt. These cavities were left as the magma hardened around trapped gas bubbles, and then subsequently filled in partly or entirely with other minerals left by several episodes of groundwater flow. Calcite, quartz, and zeolites are all to be found in the Hampden Basalt cavities; the variation in minerals is evidence for multiple episodes of mineral formation.
On the basin margins, streams coming down from the adjacent hillsides deposited thick sequences of poorly sorted, poorly weathered sands and gravels in alluvial fans and deltas. Multiple cross-cutting channels are visible in the roadcuts and natural exposures of this unit. Contacts at the base of individual channels are often zones of preferred groundwater flow through the unit. These rocks, generally stained bright red by the mineral hematite, can be seen in many areas around greater Hartford, particularly along Interstate 84 and adjacent highway onramps and offramps to the east of Hartford. This Portland Arkose, as it is called, lies stratigraphically on top of the basalts and lake beds shown above, and is the youngest (and thickest) of the units in the Hartford Basin sequence. (See figures at left and below.)
In the image above and to the left, Dave Mostoller '94 and other members of the first Hartford field trip crew study the cross-bedded gravels of the Portland Arkose, the uppermost immature sand and gravel unit in the Hartford Basin sequence. In the photo below left, Debbie Schwartz, Anna Randall, Kevin James and Walid Hamzi are checking out some of the bedding structures in the Arkose, and below right, Sarah Kopczynski reacts to one of Walid's numerous humorous commentaries on some of the unusual things he's finding. The alternating beds of sandstone and conglomerate reflects variations in the ancient sedimentary environment, as sands and gravels were laid down in the bed of this Jurassic stream.
This roadcut exposure, available for study in 1998, was only being cut by highway construction during the earlier trips to the Hartford Basin. (These last two photos both by Dr. John Graham, currently of Lander University in South Carolina.)

[Many thanks to Dr. Pranoti Asher, now of Georgia Southern University, who planned and coordinated the first Hartford field trip for Colby. Except as otherwise noted, all photos above are by Mr. Bruce Rueger of Colby. Funding to support this trip is provided by the Thomas Bove Memorial Fund and the Geology Alumni Fund at Colby College.]





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Last modified 25 April, 2000