Take a Walk Through the Geological History of Manhattan at Hernshead
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Take a walk through the geological history of GPS coordinates for relevant outcrops: Manhattan at Hernshead in Central Park Graphic granite - N40°46.650, W073°58.365 Vein quartz - N40°46.650, W073°58.373 Granofels - N40°46.654, W073°58.373 Metapelite - N40°46.653, W073°58.370 Biotite-garnet schist (amphibolite) - N40°46.655, W073°58.363 Fold hinge - N40°46.645, W073°58.374 Decompression cracks - N40°46.662, W073°58.358 Note: all GPS datum is record using WGS-84 in degrees, decimal minutes The information and data presented in this document were created by the graduates of the Masters of Arts in Teaching (MAT) program at the American Museum of Natural History in 2016. Among the goals of this research program is the production/development of engag- ing and educational exercises in Earth Science that can be conducted in the urban Manhat- tan environment. Contributors to this work include Arthur Funk, Hali Englert, Sean Krepski, Caitlan Tully, Nicholas Tailby, George Harlow, James Webster and Denton Ebel. Collection of soil, rocks, artifacts, plants or their parts, animals (including insects) or their nests or eggs, from NYC Parks is strictly regulated. Persons wishing to make such collections within Parks’ property must receive special permission to do so under a NYC Parks Research or Wildlife Permit. page 1 page 2 Directions to Hernshead outcrop (follow the footsteps) Field stops at Hernshead (i) Exit the American Museum of Natural History from the main entrance on Central Park West (see point 1 ) Once arriving at Hernshead, you can use the map attached below to investigate the various geological features at this location. (ii) Walk downtown until you reach the 77th St entrance to Central Park (~160 meters) If you have a GPS or smartphone, you can use GPS coordinates provided on page 2. If you do not have a GPS or compass, we (iii) Enter Central Park at 77th St and walk to the junction with West Dr (~160 meters) recommend you orientate yourself with the outcrop outline provided in the gure below in order to nd key features. Note that (iv) Cross West Dr and head south until you intersect the rst walkway to your left (~15 meters) specic eld stops (with numerical value and blue box) are listed in the gure below and are presented in the corresponding, later (v) Follow the path until you reach the Ladies Pavilion and outcrop at the edge of “The Lake” (indicated by star below; ~75 sections of this eld guide. meters) St St (image modied after Taterka, 1987) th nd Figure key 4 E 79 5th Ave E 72 Metapelite Map key The Metropolitan American Museum Granofels 1 Museum of Art 7 of Natural History Quartz vein (2nd generation) 2 Hernshead outcrop Garnet-biotite schist (amphibolite) o 3 Bethesda Fountain Quartz vein (1st generation) 3 4 Cherry Hill Fountain 8 3 1 Field stop 5 Strawberry Fields 6 7 6 Delacorte Theater 6 7 Great Lawn 4 8 Loeb boathouse The Lake Walking path 2 to Hernshead 5 1 Lake or watercourse Rock outcrop Central Park West 5 St St St St st Stone/concrete th th nd structure 1 W 81 W 86 W 77 North W 72 Major road 2 North Footpath 400 800 1600 6 12 The American Museum meters meters of Natural History page 3 page 4 Depositional environment of the Stratigraphy in Manhattan (with a focus on Hernshead metasediments at Hernshead Paleoreconstruction of Laurentia The metasedimentary units observed at Hernshead are Simplied stratigraphy of Manhattan The main metamorphic units observed at Herns- part of a large allocthonous block (e.g., a volume of rock head were originally deposited as marine sedi- North Pole moved from the locus of formation via a low angle thrust ments (e.g., detrital material at or near the con- fault) that was tectonically displaced during the Taconic ЄOh-bz tinental margin) over a timespan ranging from Orogeny. This movement of material likely involved nu- the late Cambrian (~500 Ma) through mid-Ordo- 460 Ma merous faults, such that the general stratigraphy in Central Cameron’s Line vician (~450 Ma). During this time period the Park is often difficult to resolve. With this in mind, it should Є strata consisted of a series of interlayered muds be noted there is some debate about the position of Cam- Om and sands that were weathering off the ancient Panthalassic Ocean eron’s line within Central Park and this is important with continent of Laurentia. regards to classifying geological units at Hernshead. A good summary of the various interpretations of the struc- Geological units ЄOi During this same period of time the Taconic ture and stratigraphy of Central Park is presented by Puffer observed at Island Arc was approaching the eastern margin et al. (2010). A great uncertainty in the geology of Central Hernshead of Laurentia, actively and cyclically producing Park is the fault-bound contact between the Manhattan Yf volcanic material that is represented by thin and Hartland schist, a contact known as Cameron’s line. mafic layers within the overall sedimentary se- Similarly, much debate exists regarding the Hartland ЄOh-bz = Hartland schist (Bronx Zoo-type) quence (e.g., the dark Fe- and Mg-rich material schist in Central Park and whether it is akin to units seen at still observed as thin units at Hernshead and at Siberia Pelham Bay Park or if it represents a different subunit ЄOm= Manhattan schist various locations within Central Park). (Brock and Brock, 2001). Taterka (1987) places Cameron’s ЄOi = Inwood marble th Line at positions between ~87 Street (Central Park West) Y At Hernshead the geological units and 82nd Street (Park Ave) which makes Hernshead a f = Fordham gneiss represent a very small section of Equator member of the Hartland schist. Merguerian and Mergueri- the larger “Manhattan Prong” Laurentia an (2004) by contrast, place Cameron’s Line close to the outcrop at Hernshead at 72nd Street (Central Park West) and 67th Street, and this c r would make Hernshead A Theoretical cross section of the Laurentian continental margin Figure key d a member of the Man- n a l and depositional environment for the Manhattan schist s Baltica hattan schist. It is worth I c i West East n noting that for thermo- o = mid-ocean ridge c a T barometry purposes = estimated outline of current continents (the focus of this guide), Iapetus Ocean unit designation is not important. Future work, SEA LEVEL = subduction zone Avalonia particularly geochronol- ogy, would do well to = locus of the Manhattan schist place accurate con- Terr straints on the position igeno us meta and relationship be- sediments = continental landmass tween the various (image modied after Scotese, 1997) fault-bound units in Laurentian continent = ocean South Pole Manhattan. page 5 page 6 Simplied Geological Map of Hernshead Folding and foliation development The outcrops at Hernshead, located immediately adjacent to the Ladies Pavilion, get their name from the out- The dominant style of fabric or texture observed in regionally metamorphosed rocks like Hernshead is a line of the rocks when looking back at the outcrop from the northeast - with the outline somewhat resem- planar fabric that is generally referred to as foliation. It is formed by the parallel or subparallel arrangement bling the bended neck and head of a hern or heron. A number of geological units, metamorphic fabrics and of platy (e.g., micas) and elongate minerals within a rock. Most other rock types we see in nature tend to later geological events are recorded at Hernshead and give clues to the overall geological history at this loca- have a random orientation, but because metamorphic rocks develop in the presence of deviatoric stress, tion. The purpose of this document is to provided the interested reader with a guide to reading and interpret- they develop a texture or foliation during the processes of recrystallization or neocrystallization. ing these features. Minerals that crystallize or grow in a dierential stress eld, like sheet silicates (e.g., micas), will grow with Figure key 42 their sheets orientated perpendicular to the direction of maximum stress (indicated by arrows in the gure o o below, where DTc denotes deformation during the Taconic Orogeny). Similarly, some minerals contained XX Bedding 66 90 o o within the original sediment may undergo dynamic recrystallization or develop subgrain boundaries in the 64 Metapelite o Vegetation (cover) presence of this stress eld, producing textures indicative of metamorphism (e.g., undulose extinction). Metagranitoid 50 o o The outcrops at Hernshead show a series of steeply dipping isoclinal folds that show a shallow plunge to the Detritus (sand/gravel) 72 74 o 72 55 o southwest. Strike and dip measurements from Hernshead show steep bedding, typically towards the north- Water o west that is repeated throughout the outcrop. In addition to folding, a micaceous fabric subparallel to the 60 o 90 55 o 50 Garnet-biotite schist (amphibolite) o o fold axial surface is observed in most geological units. Quartz vein 38 o 76 o 57 o Fold axial surface 58 o o 50 o 30 o 50 o 80 72 o Folding at and foliation at Hernshead are 37 o intimately related to the stress eld that existed during the Taconic Orogeny. Most 44 o of the fold axial surfaces and foliation pat- 60 o terns indicate that the maximum principle o 50 stress (σ ; which can be considered the 70 o 1 o 90 82 direction of maximum shortening) is a o 62 o northwesterly-southeasterly direction (e.g., 035O-215o). 60 o 65 o 60o 55 o 65 o D o North plane vertical 85 horizontal plane Tc 20 ft 40 ft 70 o page 7 page 8 1 Folds and their orientation at Hernshead 3 ft 6 ft The rocks of Central Park record a complex history of continental growth through sedimentation and tectonic processes that spans millions of years and multiple orogenic events.