Problem+4+-+Rifting

Part 1 - Begin at the Beginning Basic continent-continent rifting

GEOFF'S ADVICE!! -- "I agree completely- the volcanics would have been basaltic lava flows, similar to the types you would see emanating from any Hawaiian-style eruption. In many areas of the CT river valley there are dike swarms and sills comprised of diabase (the intrusive equivalent of Basalt) that cut through the country rock as well. The Palisades sill in the Newark basin is a diabase sill...not quite erupted, but not quite a plutonic rock like a gabbro. These magmas worked their way up close to the surface, and in some cases were erupted, but for the most part they exploited seams in stratigraphy and formed dikes and sills. Some of these have been exposed through weathering and erosion.

Now, you probably know that the entire East coast was torn apart during the Triassic, and that a number of rift-related basins with significant volcanics opened up roughly parallel to the current continental coastline. In places like the Bay of Fundy (Canada), the Hartford Basin (CT.) and all the others the attenuation of the lithosphere would have provided migratory pathways for magmas to push up to the surface. This is analogous to what is happening in the Rio Grande Rift in New Mexico. In some areas, the flows would have entered water which would explain the presence of pillow basalts and lava flows with extensive zeolitization ( = full of zeolite minerals). In other places, normal pahoehoe and a'a lava flows would have been the norm. Now, I am pretty sure, but obviously you guys should look this up, but I think that the compositions that were erupted were pretty much on the mafic end of the scale (basalt) with very little evolved stuff.

Keys in stratigraphy- you will probably see lava flows comprised of multiple units. In some cases, there may be inter bedded sedimentary units if there was a lull in eruptive activity in an area and sedimentation occurred. "



[] Paleomagnetic Differentiation and Correlation of the Late Triassic Volcanic Rocks in the Central Appalachians (with special reference to the connecticut valley). "In Connecticut four periods of late Triassic-early Jurassic volcanic activity have been dis­tinguished geologically. They are from old to young: the Talcott, Holyoke, Hampden, and Higganum volcanic events. The mean inclination of the T.R.M. is + 12° for the Talcott, + 25° for the Holyoke, + 42° for the Hampden, and + 35° for the Higganum volcanic units. This paleomagnetic stratigraphy enables one to differentiate between the volcanic units and to correlate the units of one basin, with contemporaneous outflows or intrusions in other basins. "

***All taken from this paper **


 * The CT valley (or rift zone) is a downfaulted crustal segment with dimensions of 5 to 20 by 105 miles. The north-south-trending segment is located between the Eastern and Western Highlands, which are chiefly composed of early Paleozoic metamorphic rocks. It is split into 2 basins, each with their own sedimentary and volcanic records, the Deerfield (smaller basin located in northern Mass) and the Hartford (larger basin located in Southern Mass and CT.)

[] Diachronous supracrustal extension in an intraplate setting and the origin of the Connecticut Valley–Gaspé and Merrimack troughs, northern Appalachians

"The Silurian/Early Devonian rocks consist of marine clastic deposits with subordinate carbonates, lava flows and terrestrial deposits. The origin of these sedimentary basins is still poorly understood. Metamorphic ages and structures in the Laurentian margin, major unconformities and syn-sedimentary normal faulting in both troughs argue for significant crustal extension during deposition"

[] Development of tectonic cyclothems in rift, pull-apart, and foreland basins: Sedimentary response to episodic tectonism.

[] stratigraphy in western new england

[] Geologic time chart of Connecticut

[] VERY basic rift model

[] Lithosphere stretching and sedimentary basins (North Sea, but we can compare)

Rift powerpoint

 Fig. 5a. Progressive formation of a rift valley through extension of the lithosphere and continental crust (by about 50 km). Note that uprise and decompression of the underlying asthenosphere results in magma formation. The crust responds by brittle fracture. Early rift sediments are downfaulted into the developing rift (graben). Erosion takes place on the sides of the rift valley. > http://images.google.com/imgres?imgurl=http://www.le.ac.uk/geology/art/gl209/lecture3/image46.gif&imgrefurl=http://www.le.ac.uk/geology/art/gl209/lecture3/lecture3.html&usg=__25mZs466gysU0GwbjBHpQO7TbVs=&h=989&w=753&sz=134&hl=en&start=8&um=1&tbnid=H308nSHNEXASXM:&tbnh=149&tbnw=113&prev=/images%3Fq%3Dcontinental%2Brift%26hl%3Den%26sa%3DN%26um%3D1__

**Rifts: The Making of a Continental Margin.** When a new spreading center forms beneath a pre-existing continent, a rift forms that will eventually, if allowed to proceed normally, divide the original continent into two, with new ocean floor being created to separate them. As the two newly formed plates begin to separate, molten material, mostly basalt, from the mantle beneath will flow upwards into the crack. The heat from this molten material is conducted to the continental material above, reducing its density and causing it to float higher in the mantle, producing a ridge of mountains above the spreading center. As spreading continues, blocks will break loose from the sides of the crack and subside into the void, creating the characteristic "rift valley," such as that in East Africa. As spreading continues, the rift valley will deepen, ultimately subsiding below sea level and allowing ocean water to fill the valley. The now water-covered subsidence blocks will later become the submerged continental shelves. Depending on the rate of spreading and the amount of heat flowing into the rift, these continental margins may be broad or narrow. Also depending on heat flow, which to a large degree depends on the proximity of plumes, volcanoes may or may not form. Surface fissure flows of basaltic lava may also occur where heat flow is high. (Plumes generally appear to be associated with the formation of new spreading centers, a notion that will be discussed further in a later lesson.) Continued spreading causes the complete separation of the two land masses, with new sea floor being created beneath the ever-widening ocean. As the continental margins move farther from the heat flow at the spreading, the mountains formed along the continental margins cool and slowly subside back into the mantle. Depending on how much material erosion has removed from their summits, they may sink below the waves and vanish forever. Freshly rifted continental margins tend to have steep walls, the continental shelves plunging nearly vertically from a few hundred feet deep to the ocean floor, many thousands of feet below. As erosional sediments are washed off the land surface, they first cover the continental shelves, then wash over the precipice to fall on the ocean floor below. Very old continental margins, therefore, tend to have large accretionary wedges on the deep ocean floor piled up against the continental margin. The rifting process is not always as 'clean' as the above description may sound. Sometimes a segment of spreading center may shift slightly while separation is occurring, causing some of the subsidence blocks to be separated from both daughter continents. These messy remnants may become submerged plateaus or 'banks' on the sea floor, coral atolls, or islands in their own right. New Zealand, for example, is a continental fragment left behind after a long forgotten rifting episode. Taken From:  [] Blanchard, Donald L. __The ABC's of Plate Tectonics.__ "Sedimentation and Continental Growth."

http://www.knowledgerush.com/kr/encyclopedia/North_American_craton/ North American Craton

-Hartford and Newark basins formed together in a "half-graben geometry"?? as symmetrically opposing basins (Manspeizer, 1988). -Basalt in this region has been labeled as Initial Pangean Rift (IPR) basalt (Puffer, 1994). -The magma that formed this basalt has the same sills found in the Gettysburg, Culpeper, and Newark basins (Woodruff, 1995). -This basalt is also present in offshore platforms such as Nantucket and Long Island (Hutchinson, Klitgord, Detrick, 1986). -Believed to be the same basalt in offshore and onshore basins in Morocco (Bertrand, 1991). -also something about these basalts being in the Lower Jurassic stratigraphic section... ...running out of battery I will cite this later haha.

a lot of good information on this site http://www.earthview.pair.com/ctriver.html

"**The Connecticut Valley originated in the Mesozoic. Pangea began to split, forming the present Atlantic Ocean. Besides the big split of the Atlantic, many smaller faults cracked the land due to the stretching stresses. These "rift valleys," similar to today's Death Valley and others of the Basin and Range, formed the initial drainage of the ancestral Connecticut Valley.**
 * The fault that dominated the Mesozoic rift was located on the eastern side of the valley, and is known as the Eastern Border Fault that can be traced from New Haven, CT. to Keene, NH. Rivers rushed into the rift valley and deposited sedimentary materials, gravel, sand, and mud. Gravely alluvial fans were major deposits along the mountainous eastern margin of the old valley, while sandy-muddy floodplain, shoreline and lake deposits dominated the lower valley elevations.**
 * One of the unique aspects of the Connecticut Valley of today is that sedimentary rock from the processes just described, is easily seen along the rocky river bends and roadsides in the MA and CT portions of the valley. Since almost all of New England is composed of metamorphic rock (due to the Paleozoic collision), these rusty sedimentary layers from alluvial fans and lake beds provide important geological examples and evidence of our Mesozoic history, including some world-class fossils.**
 * A sedimentary feature that is unique to the Connecticut Valley is the armored mud balls found in Turners Falls, MA and vicinity. Armored mud balls formed in the Mesozoic sedimentary layers as streams rolled balls of hard mud downstream. The mud became round as well as soft and sticky on the outer margin, allowing sand and pebbles to become attached (the armor). The balls were quickly buried by other stream deposits and eventually lithified. Lithified armored mud balls have only been found in about 10 other localities in the world, in old beach deposits. The Turners Falls area armored mud balls are the only stream-formed armored mud balls in the world. (Little, 1982) Excellent examples of these forms are preserved in boulders placed along the river at Unity Park, Turners Falls, and in the Greenfield Community College "Rock Park".**
 * Lava flows are dramatic and important Mesozoic events in the Connecticut Valley and profoundly influence the landscape today. The dark basalt lavas, called "traprock", flowed out over the Mesozoic lowlands, commonly reaching over 100 feet in thickness. Today these flows, tilted by movements along the ancient Eastern Border Fault and then exposed by erosion, form spectacular ridges that stretch tens of miles, creating interesting, dramatic vista points and important upland ecosystems in the middle of the wide valley (Fig. 5). Examples include the Pocumtuck Range (Greenfield - Deerfield, MA) and the Holyoke Range that trends east-west about 10 miles from Amherst to Easthampton, and then southerly for about 60 miles (known as the Metacomet Ridge) to the outskirts of New Haven.**
 * The basalt flows exhibit interesting geologic features such as pillows, formed by flow underwater, and columns, created by contraction cracks during cooling. The basalt is an important geologic resource, quarried for crushed stone and rip-rap.**
 * By the end of the Mesozoic Era, 65 million years ago, the Eastern Border Fault had been inactive for about 70 million years allowing the valley to become completely filled with sedimentary deposits. Surrounding areas were smoothed by erosion and all became part of a peneplain, an erosional plain of regional extent. Several high places resisted the forces of erosion, and are known as monadnocks, named for a prominent example of this feature, Mt. Monadnock of southwestern NH.**