Portland Formation | |
---|---|
Stratigraphic range: Hettangian-Sinemurian ~ | |
Type | Geological formation |
Unit of | Agawam Group, Newark Supergroup |
Sub-units | Turners Falls Sandstone & Mount Toby Formation |
Overlies | East Berlin Formation |
Lithology | |
Primary | Sandstone |
Other | Mudstone, siltstone, limestone |
Location | |
Coordinates | 42°18′N 72°30′W / 42.3°N 72.5°W |
Approximate paleocoordinates | 24°00′N 18°36′W / 24.0°N 18.6°W |
Region | Connecticut, Massachusetts |
Country | USA |
Extent | Deerfield & Hartford Basins |
The Portland Formation is a geological formation in Connecticut and Massachusetts in the northeastern United States.[1] It dates back to the Early Jurassic period.[2] The formation consists mainly of sandstone laid down by a series of lakes (in the older half of the formation) and the floodplain of a river (in the younger half). The sedimentary rock layers representing the entire Portland Formation are over 4 kilometres (2.5 mi) thick and were formed over about 4 million years of time, from the Hettangian age (lower half) to the late Hettangian and Sinemurian ages (upper half).[3]
In 2016, the paleontologist Robert E. Weems and colleagues suggested the Portland Formation should be elevated to a geological group within the Newark Supergroup (as the Portland Group), and thereby replacing the former name "Agawam Group". They also reinstated the Longmeadow Sandstone as a formation (within the uppermost Portland Group); it had earlier been considered identical to the Portland Formation.[4]