Field Trip Report

Orchard Beach, Pelham Bay Virtual Field Trip

Where are we going?

• Orchard Beach at Pelham Bay Park in Bronx, Northernmost borough of NYC

• From the parking lot, walk on the gravel path past tennis courts toward the sound, then walk NE along the beach to Twin Islands

• This 10-20 minute walk will include several stops

What are we going to see?

• Hartland formation: exotic terrane (islands off the coast of Africa) that collided with North America ~440 ma

• Devonian intrusions into the Cambro- Ordovician rocks

• Evidence of glaciation (Wisconsin ice sheet, 1000 ft thick in this part of NY, ~11 ka -80 ka)

• Sediments that were metamorphosed during Taconic and Acadian orogenies

• Quartz-feldspar gneiss, biotite-sillimanite schist, amphibolite

• Glacial till, glacial erratics, striations, outwash plain, terminal moraine in the distance

Map of NYC geology – US geological Survey

Tectonic evolution of the east coast:

• Follow along with this cartoon version of the area’s tectonic history in your handout

• Overview: • Grenville Orogeny completed the assembly of

Rodinia, ~1.5-1 ba • Post-Grenville rifting created Iapetus ocean • Iapetus ocean started closing • Three pulses of Appalachian mountain building,

Taconian, Acadian, and Alleghanian orogenies, close Iapetus Ocean

• Pangea breaks up in the Mesozoic, rifting creates Atlantic ocean

• In this field trip we have glimpses into two of the three pulses of Appalachian mountain- building during early stages of Iapetan closure: the Taconic Orogeny (Cambro- Ordovician), and the Acadian Orogeny (late Devonian)

Adapted from Earth: Portrait of a Planet by Steve Marshak

Glaciation and 1st stop

• The top stratigraphic layers are much younger than the tectonic events described in the previous slide

• Ice age in the Pleistocene shaped landscape, modified drainage, and eroded strata

• Last advance of ice: Wisconsin stage of the Laurentide ice sheet

• Terminal moraine at the edge of the ice sheet creates Long Island

• Long Island Sound was a glacial lake; as climate warmed and sea level rose, the outwash lake became an estuary then a sound; tall points of the moraine are now islands

2nd stop: Hartland formation

• Hartland formation: exotic terrane (islands off the coast of Africa) that collided with North America ~440 ma

• Sediments that were metamorphosed during two of the three pulses of Appalachian mountain building: Taconic and Acadian orogenies (check tectonic evolution cartoon in your handout)

• Quartz-feldspar gneiss, biotite- sillimanite schist, amphibolite

• Follow the links; hope you can hear me over the wind!

• https://youtu.be/aXcqGEgAH2k

• Severe deformation

• Partial melting of the schists and gneiss produced abundant quartz- feldspar leucosomes

• Leucosomes: lenticular shape, coarse- grained, variable thickness, high-grade metamorphism product

• Garnets: metamorphic index mineral

• https://youtu.be/j8YZUQWN1j4

Close up of garnet porphyroblasts

Cavities left by dissolution of calcite in amphibolite

Folded leucosomes in the Hartland formation

Glacial landscape

• Glacial grooves and striations in Hartland formation

• https://youtu.be/U8yFnGqG9A0

• Unconformity: glacial till overlying the Hartland formation

• https://youtu.be/tq-sIopV3Uw

Glacial erratics at Orchard Beach

Devonian dikes with straight planar boundaries, unaffected by tight folding that deformed Taconian leucosomes

• https://youtu.be/NzwoYABulsw • https://youtu.be/UWDHv9_ogCI

Note glacially formed Long Island Sound in the background; Long Island, across the sound, is a terminal moraine

Cross-cutting relationships – Devonian dikes intrude Cambro-Ordovician Hartland rock

Metamorphosed turbidites

• Rhythmically bedded sequences of gneiss and schist occur; these are interpreted as turbidites, deposits from sediment-laden flows spewed into deep water. On the northwestern side of North Twin Island, graded bedding is preserved, allowing us to deduce the direction of stratigraphic tops

• https://youtu.be/z1Tl1BnZVho

• The last hurrah – more cross- cutting relationships:

• https://youtu.be/iJAVnPGg5v0

Thank you

• To my son Lev for his phone camera wielding skills • To all of you for coming along on this virtual trip with me

• References: • https://pbisotopes.ess.sunysb.edu/reports/ny-city/ • http://geologycafe.com/nyc/common/captions.htm • https://blogs.agu.org/mountainbeltway/2010/03/25/transect-debrief-5-

sedimentation-continues/ • http://www.geo.hunter.cuny.edu/courses/geog260/PP5-2011Glaciation.pdf