Hartford Basin Long Island Geologists Field Trip June 27, 2002

The Hartford Basin is one of the many rift basins along the east coast of North America that formed during the early stages of rifting of Pangea during the Triassic and Jurassic. The rifting of Pangea eventually led to the formation of the Atlantic Ocean. The Hartford Basin contains a thick sequence of stream and lake sediments and basaltic sills and lava flows all of which are well exposed. We will have lunch at Dinosaur State Park in Rocky Hill Connecticut where you will see five hundred dinosaur tracks enclosed within the Exhibit Center's geodesic dome.

You may wish to visit some of these sites that give geological information pertinent to the Hartford Basin

Stop 1. Off exit 10 on Interstate 91 on Route 40 going toward Mt. Carmel and Hamden. We will examine a large exposure of an alluvial plain sequence of sandstone and conglomerate interbedded with floodplain mudstones and well-developed caliche profiles in the New Haven Arkose. This is a braided stream (alluvial plain) sequence of pale red, channel sandstone and conglomerate interbedded with a small fraction (14%) of flood plain mudstone in the New Haven Arkose. Caliche, soil calcite, commonly found in the B soil horizon in semi-arid climates is found at the tops of mudstone sequences as well as in some of the channel sediments. The braided streams were draining to the southwest. The rivers were ephemeral, with large fluctuations in water discharge associated with flash floods. There was a high gradient and the streams carried a coarse bedload of sand and gravel. The flood plains were exposed for periods long enough for soil horizons to develop before the next channel would erode the sandy mud down to the caliche layer and then deposit channel sands and gravels . Note that the caliche occurs as independent nodules, around roots and occasionally as more massive units.

Stop 2. Dinosaur State Park at Rocky Hill, Connecticut. At the Dinosaur Park please note in the displays the environments of the setting of the Hartford Basin. In the background of the large display you can see the mountains to the east with the alluvial fans forming in front. The mountains are east of the border fault and in the eroding basement. The alluvial fans are the settings for the Portland Arkose at Stop 5. The streams and lakes are the setting for the East Berlin Formation. In front of the building there is an outcrop showing a perennial lake sequence.

Stop 3. Northeast side of Highway 9 just before exit 22 in Berlin Township. We will see a sequence typical of a perennial lake setting in the East Berlin Formation. A typical sequence consists of pyritic black shale that accumulated in a deep, anaerobic part of an alkaline lake. These beds sometimes contain fossil fish remains. The gray mudstone on either side of the black shale formed in shallow water and commonly has ripple marks, mud cracks and dinosaur footprints. Sandstone is commonly found near the top and bottom of the cycles and is indicative of shallow water near the shore. It is thought that the lakes essentially covered the rift valley and periodically increased and decreased in size perhaps due to Milankovitch forcing (Olsen and Kent, 1996). Based on the pollen the climate was humid with annual wet and dry seasons typical of the tropics.

Stop. 4. East side of Route 5 south of Hawthorne Inn (2387 Wilbur Cross Parkway, Berlin). We will see a sequence of vesicular pillow basalts. The pillows often have sediment or shattered volcanic debris between them. Pillow basalt forms under water. The pillows are actually tubes through which the lava flowed. The outer rim cools very rapidly and the lava flows through the center continually breaking through the cooled front. Most two-dimensional slices through a tube produce a pillow shape.

Stop 5. Intersection of Routes 17 and 77 south of Durham Center. At this location we are close to the eastern border of the Hartford Basin where there is coarse sandy conglomerates of the Portland Arkose. This sequence formed in an alluvial fan along the Eastern Border Fault. At this stop the Eastern Border Fault is only 0.6 kilometers to the southeast. Boulders up to 60 cm in diameter are found here. Sedimentation was episodic characterized by flash floods after heavy rains. The conglomerate would be transported and deposited during the peak of flow and the interbedded lenses of sandstone were deposited during the waning stages of flow. The absence of mud suggests that debris or mudflows were not a factor in the formation of these deposits.

Stop 6. 0.25 miles west of exit 59 off the Wilbur Cross Parkway is an outcrop of conglomerate of the New Haven Arkose unconformably overlying the Paleozoic Maltby Lakes metavolcanic rocks on the western margin of the Hartford Basin.

	Stop 1 New Haven Arkose Outcrop of lighter color channel gravel and sand with darker red mud layers		Stop 1 Nodular caliche
	Stop 1 Possibly caliche developed around roots.		Gravelly arkosic channel sand overlying dense array of caliche nodules developed in mud overbank deposit
	Stop 2 Dinosaur State Museum Diorama looking toward east showing setting at time of deposition of East Berlin Formation. Mountains in background are along Eastern Border Fault.		Stop 2 Another diorama looking toward east at time of deposition of East Berlin Fm.
	Stop 2 Dinosaur tracks		Stop 3 East Berlin Fm. Typical perennial lake cycle.
	Stop 3 Deformation in black shale		Stop 3 Fossil wood in black shale
	Stop 4 Pillow Basalt		Stop 5 Sand layer within conglomerate Portland Arkose
	Stop 6 Unconformity Thin layer of basal conglomerate of Portland Arkose overlying metavolcanic rocks of the Maltby Lakes Fm.