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The Geologic History of Weeks State Park by J Panoramic view from the summit of Mt. Prospect Mt. Cabot Mt. Waumbek Mt. Washington Cherry Mtn. S. Twin Mt. Lafayette Dalton Mtn. Mt. Pleasant Goback Mtn.Cape HornPercy Peaks Drone image courtesy of the Bates College Image Center The Geologic History of Weeks State Park by J. Dykstra Eusden Bates College Geology and Woodrow B. Thompson New Hampshire Geological Survey LiDAR image of Weeks State Park area. The lookout tower on top of Mt. Prospect is in center of image. 1:11,000 Weeks State Park Association The Bedrock Geology of Massachusetts border all the way up the Weeks State Park Connecticut River valley and ends just a bit beyond Weeks State Park. In the Park it is a zone Weeks State Park is a fantastic place to get about 100 meters wide of crushed, fractured, a glimpse of the geologic forces and events somewhat rusty weathering, crumbly rock. that have shaped New England and the White Quite often veins of pure quartz are found Mountains. Major faults are revealed here, as within and adjacent to the mapped fault. These well as a variety of rocks that span 500 million were once geothermal fluids flowing along or years. For a relatively small area Weeks State near the fault that ultimately re-precipitated as Park reveals great geology that is accessible for quartz veins (Figure 1). Adjacent to this narrow all to explore! A glossary of italicized terms can be found at the end of this brochure. 1 Ammonoosuc Fault The most dramatic feature of the bedrock geology at Weeks State Park is certainly the Ammonoosuc Fault, cutting northeast right through the center of the park. This was a normal fault where the southeast side moved up relative to the northwest side which moved down. It has remained inactive and unmoving for the last 80-90 million years since the waning stages of the breakup of the supercontinent Pangea and the associated rifting to create the Atlantic Ocean. One of the paradoxes at Weeks State Park is that the down-dropped side of the fault has the highest topography culminating Figure 1. Quartz vein intruding Lost Nation in Mt. Prospect. Typically you would expect the gabbro. Hydrothermal fluids filled cracks that surround and are within fault zones. up-thrown side to be the topographic high, but here it is reversed. The bedrock on the northwest side of the fault, composed of the zone the rock is still heavily fractured giving Lost Nation gabbro and Albee Formation, was Weeks State Park its characteristic and unique probably more resistant to weathering than broken talus surface that the forest has now the rocks of the Ammonoosuc Volcanics and overgrown. All these fractures are conduits Oliverian granodiorite on the southeast side. for bedrock groundwater flow and allow the Glacial ice flowing northwest to southeast over water to interact with a greater surface area of Mt. Prospect plucked away the softer rocks the bedrock than is typical in regions without on the southeast side of the fault leaving the faults. The bedrock geochemistry of the Lost present topography. Nation gabbro and the Ammonoosuc Volcanics is strongly calcium-rich due to the presence of The Ammonoosuc Fault extends from the white crystals of plagioclase feldspar in these Page 2 Bedrock Geologic Map Exofpl aWeeksnation State Park ^ quandartz v Vicinity,ein 62 o G ² bLancaster,asalt dike NH $+ Olnpg by ") J. DykstraOln Eusden and Kurt Niiler ¹ Dept.Oa ofm Geology,a Bates College Lewiston, ME o Cal 2018 line of cross section Cal 39 ¹ BedrBedrockock Un Unitsits on Map Mesozoic Ammonoosuc Fault 2¹2 afz Zone 37¹ Ordovician Lost Nation pink Oln 4 ") Olnpg granite ") Oln$+pg A’ ") G") ") Ordovician Lost Nation gabbro ¹36 Oln ") ") Oobg Ordovician Oliverian biotite 3 ") ^ granodiorite 5o3 ^ $+ 56 o Olnpg o 80 Oama Ordovician Ammonoosuc G ") Oama Volcanics Amphibolite ") ") ") Cambrian Albee Formation ") ") Cal ") Ammonoosuc Fault ") ") Quartzite ") ") 30 G") ") ¹ ") o59 ") ") ") 58 ¹26 Cal")") o ") ") ¹38 6 ExpSymbolslanatio nused on Map ") ") ") ") Explanation ^ G 5 quartz vein outcrop location ") afz ^ $+") ") 2 quartz vein G ") basalt dike outcrop location ") 45¹ G basastrikelt di kextensione of basalt dike ") 7 32¹ $+ ") Olnpg A $+ Olnpg outcrop location ") 1 ^ ") Oln ") G ") ¹ Oln outcrop location ") ") ¹ Oama strike and dip of foliation o Oama o Cal Oobg Cal strike and dip of bedding line of cross section line of cross section Bedrock UniAt -s A’ Miles Bedroseeck diagram Uni att sbottom center 0 0.25 0.5 1 afz afz 1 Olnpg figure locality, keyed to text Olnpg Oln Geologic Cross Section Oln Mt. Prospect Oobg Oobg Oama Cal Oama A’ Oama Cal A Cal 1100 - Olnpg 1000 - 900 - 800 - Oobg 700 - Oln 600 - afz 500 - 400 - Elevation in Feet Elevation 300 - Page 3 rocks. The calcium can more readily leach out into a dark green colored rock with white specks of these rocks as a result of the Ammonoosuc of plagioclase. In places the Lost Nation gabbro Fault, producing soil conditions conducive to a chilled against blocks of rock that it enveloped variety of calcium-loving rare plants in the park, while intruding into the region. In these places such as Dutchman’s Breeches. we see a decrease in grain size near these blocks making the Lost Nation gabbro even more Lost Nation Gabbro dense and hard. The Lost Nation pluton comprises the bulk of the bedrock northwest of the Ammonoosuc Fault Albee Formation and formed in the Ordovician geologic time There are two areas labeled “Cal” on the map: a period, about 450 million years ago. The main small one in the center of the map and a larger unit is an igneous intrusive rock that froze from a one extending from there all the way to the magma deep under ground. Its geologic name map’s northern border (Figure 3). Previous is a gabbro, an equivalent to an extrusive basalt, and made of the minerals calcic plagioclase 3 feldspar, pyroxene, olivine and magnetite (Figure 2). The tectonic setting for the Lost Nation 2 Figure 3. Albee Formation showing faint sedimentary bedding oriented vertically. This quartz rich rock was a marine sediment in the Cambrian, 500 million years ago. Figure 2. Xenolith of pink granite within Lost geologists have mapped these as the Albee Nation gabbro. The granite was picked up Formation, a 500 million year old rock from the when the gabbro intruded. Cambrian geologic period. These are former sedimentary rocks of sandstone that were pluton was a magma chamber associated with deposited in an ancient marine setting. The a volcanic island arc over a subduction zone Albee Formation is very quartz-rich and because where one plate descended beneath another. it has been metamorphosed in the Acadian The gabbro was metamorphosed during Orogeny, it is classified as aquartzite . It would the Acadian orogeny, or plate collision, in the not contribute any unusual geochemistry to the Devonian period about 400 million years ago. bedrock groundwater and as such would have The metamorphic heat and pressure turned it typical hard scrabble New England soils over it. Page 4 Radiometric age dating would be needed basalt dikes with lengths of up to maybe a 100 to confirm that these regions are indeed the meters. These are igneous rock that crystallized Albee Formation. There are some characteristic near the surface and as such are called extrusive. features of the typical Albee Formation seen The dikes are only found on the northwest side elsewhere that are missing in the blocks at Weeks of the Ammonoosuc Fault and only within the State Park. For example, normally the Albee is Lost Nation gabbro. It is possible that they are complexly folded and highly metamorphosed, contemporaneous with the gabbro but it is also but here it seems to have escaped that. Perhaps possible that they are younger, perhaps Jurassic the enveloping Lost Nation gabbro shielded it in age or about 200 million years old. Most from the deformation and metamorphism that dikes in the Northeast are of that vintage having affected the Albee Formation elsewhere. formed during the rifting of Pangea. The best place to see a basalt dike is in a road outcrop Pink Granite and Basalt Dikes just between the lower and upper parking lots There are two other minor rock types northwest near the summit of Mt. Prospect. Xenoliths, or of the Ammonoosuc Fault. The first is a pinkish- fragments of Albee and Lost Nation gabbro, can green, coarse grained granite that occurs in two be seen embedded in the basalt (Figure 5). small oval shaped patches on the map (Figure 4). It can be seen best on the Weeks State Park ski 5 4 Figure 5. Basalt dike with xenoliths of Albee Formation and Lost Nation gabbro. As the Figure 4. Rare, pinkish green granite phase basalt magma intruded it picked up pieces of of the Lost Nation. The coarse pink grains are the older rocks. potassium feldspar, the green plagioclase feldspar, and the dark glassy mineral is quartz. Ammonoosuc Volcanics and Oliverian Biotite slopes in the island of woods between the two Granodiorite trails. It is also an intrusive igneous rock, most On the southeast side of the Ammonoosuc fault likely the same age as the Lost Nation gabbro. are two very different rocks with exceptional The second rock is seen on the map as a structures in them. The smaller patch in scattering of red + symbols and two short red green on the map is the 460 million year old straight lines. These are narrow, 1-2 meter wide Ordovician Ammonoosuc Volcanics (Figure 6). Page 5 6 7 Figure 6. Ammonoosuc Volcanics with characteristic variations in color and grain size. The coarser rock on the right is composed of white calcium plagioclase feldspar and black amphibole.
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