On this trip will visit some locations within the Hartford basin, a large Mesozoic structure filled with Triassic and Jurassic sediments and basalts.
This is one of about 30 known Mesozoic basins (topographic depressions that collected sediment) in eastern North America, from North Carolina to Nova Scotia. About half of the basins are exposed, and the rest are buried under younger sediments of the Atlantic Coastal Plain and continental shelf. Basins started forming in the Middle to Late Triassic Period as early as 225 m.y. ago, and they stopped being locations for preserving sediments in the Early to Middle Jurassic Period, up to about 175 m.y. ago. To review the period names and dates, check the Geologic Time Scale.
It is widely believed that the formation of these basins and lava flows is closely linked in some way to the splitting of Pangaea, the worldwide "super continent" that existed in the early Mesozoic, to open the central part of what is now the Atlantic Ocean. What is now southern Morocco was adjacent to present-day New England (its continental shelf anyway). See the map of this part of Pangaea. The age of the lava flows has recently been determined to be near 201 m.y. ago, and the ocean opened around then or maybe later (there is new dispute about this). The basins all have major faults along one side, which caused the basin to drop down and tilt its sediments and lavas down toward that side. There might have been many big earthquakes. Such movements of the earth and its plates are a branch of geology called "tectonics."
The Mesozoic sediments are mostly "terrestrial" or formed on land, with some being "fluvial" or formed in rivers, and other layers are "lacustrine" or formed in lakes. At that time the area was much closer to the equator, but it was fairly dry as well (something like present Mexico). We will examine some of the sedimentary features that indicate origins. The lava flows are called "flood basalts" because they poured across the land from enormous volcanic fissures, now represented by dikes like the Higganum dike (trip 1). There are three large lava flows in the Hartford basin, which are identical to three found in similar basins in New Jersey and northern Virginia. McHone's suggestion (see his 1996 reprint) is that they are all portions of the same gigantic lava flows. Here is a cartoon of the Hartford basin and its formations, west (left) to east (right):
In the cartoon, the yellow layers are sandstones and mudstones, while the orange layers are basaltic lava flows. The Tr (bottom yellow layer) is the Triassic New Haven formation; the bottom orange layer is the earliest Jurassic Talcott basalt; next yellow layer is the Shuttle Meadow formation; middle orange layer is the Holyoke basalt; next yellow layer is the East Berlin formation; top orange layer is the Hampden basalt; top yellow (J) layer is the Portland formation. Feeder dikes (fissures) for these lava flows are shown in red. (1) is the Higganum dike (feeder to the Talcott); (2) is the Buttress dike (feeder to the Holyoke); (3) is the Bridgeport dike (feeder to the Hampden). The Seaward-Dipping Reflector (SDR) is a gigantic section of volcanic basalt that is wedged between the continent and Atlantic Ocean crusts.
Trip 2. Start at Dinosaur State Park (see their website), in Rocky Hill off I-91 exit 23.
Stop 1. Dinosaur trackway in the East Berlin formation. This was once a mud flat that was covered with fresh mud soon after the animals walked across it, and their tracks were preserved as casts when the mud hardened. Because bones are almost never found with tracks, separate species names have assigned to "track animals" that are gradually being correlated with actual dinosaurs. A likely candidate is pictured in the wonderful new mural by William Sillin (although it seems unlikely that the eastern mountains were so high and rugged as depicted). The trackway was found 35 years ago when the state highway dept. started to build a state garage. Sidney Quarrier and other Ct. Geological Survey employees scrambled to stop construction and get the trackway preserved as a state park. Like our other state parks, there is very little public money for people and maintenance, and a volunteer organization is responsible for helping out. There are other dinosaur trackways, but this is one of the best anywhere.
Tracks called Eubrontes cover the floor of the Dinosaur State Park museum.
Drive south on Rte 99 to Middletown, cross the Arrigoni Bridge to Portland.
Stop 2. Brazzos brownstone quarry, Portland. Brownstone is the trade name for the Portland formation arkosic sandstone ("Portland arkose"), which is made of quartz and feldspar sand with calcite and hematite cement (the rusty hematite gives it its color). Brownstone has been quarried here since the 18th century, with a production peak in the 1880's when this quarry supplied stone for hundreds of buildings in NYC and many other places. Closed since the 1930's, a section has been reopened recently by a small company that supplies stone for renovations and new buildings. Tracks from this quarry adorn the interior walls near the Science Library at Wesleyan University.
The old brownstone quarries are mainly filled with water today.
Head east on Rte 66 to Rte 17, then north about a mile or so.
Stop 3. Portland golf course conglomerate. This conglomerate is made of a mix of boulders and cobbles that washed down into the basin from steep stream valleys cut into the face of the eastern border fault (not far to the east of here). This is also called a "fanglomerate" because of the fan shape to the outwash of the stream. Similar features are common in the Basin and Range province out west. We can study the cobbles to determine what rock units made up the surface geology at that time, and some of the rock types are no longer present in the region.
Back to Middletown, north on Rte 9, exit onto Rte 372.
Stop 4. Rte. 372 roadcut between Rtes. 9 and 5/15, East Berlin. This is the East Berlin formation again, but here you can see the contact with the overlying Hampden basalt, the youngest of the lava flows. Dinosaur tracks have been found in loose boulders along the road bank, but the highway dept. will not be happy to see you taking them away!
The Hampden basalt is named after the town in Massachusetts where it was studied.
South on Rte 5/15, west on Rte 691, exit to park in Meriden and up road to ridge top.
Stop 5. Castle Craig, hanging hills of west Meriden. Here we sit on Holyoke basalt, which because it is tilted eastward and resists erosion, has west-facing cliff edges sticking upward within the basin. The lava layers are broken by faults within the basin, so that there are many hanging hills of the same Holyoke flow visible in different places. The Talcott and Hampden are thinner flows and do not seem to make such good cliffs.
The Hanging Hills are basalt ridges, the edges of 20-m.y.-old lava flows.
West on Rte 691, east on I-84, exit to Rte 322 west, right (north) on Mt. Vernon Road, past Mt. Southington ski area and left onto Roaring Brook Drive. Walk down trail out of cul-de-sac to Roaring Brook. This visit is by special permission of the land owner, Mr. John DiMaio (not open to the public).
The unconformity exposure shows the original land surface of 225 m.y. ago.
Stop 6. Roaring Brook location, Southington. This is the famous "Great Unconformity," which shows the contact between the base of the Triassic New Haven arkose and Ordovician Harrison gneiss ("Pumpkin Ground" spotted gneiss).