Abstract
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Between the Late Silurian (421 – 408 mya) and Middle Devonian (387 – 374 mya) a fluctuating shallow sea covered most of eastern North America including New York state.  LaPorte refers it to as the Helderberg sea, after the Helderberg Group of geologic formations that were deposited on its floor (1969: 115).  This shallow sea migrated northward and westward as large amounts of sediment eroded into it from the newly formed Appalachians and Catskills to the east.  Fossil evidence and examination of the lithology and primary structures of the formations within the Helderberg Group support LaPorte’s belief that the units represent contemporaneously laid deposits from different geographic sections of the shallow sea.

During the Silurian period large glacial formations melted as the world’s climate warmed following a short ice age that occurred between 445 – 429 mya (Crowell: 1999).  The melted glacial water contributed to a substantial and significant rise in the levels of major seas, creating many new marine habitats including the Helderberg Sea of eastern New York (UC Berkley 2001).

At this time New York, along with much of North America, Greenland, Ireland, Scotland, and parts of Russia, belonged to the paleocontinent called Laurentia (Fig. 1).  Laurentia was located within 30° north and south of the paleoequator (Britannica 2001).  The south shore of New York’s Hudson Bay lay at the center of Laurentia, placing most of New York state within 10° south of the equator (Fig. 2).  Thus during the Late Silurian, New York was very warm and moist – a tropical/subtropical environment.

 Fossils of stromatoporoid corals, tabulate corals, and small solitary rugose corals found in the facies of the Coeymans and Manlius formations (LaPorte 1969:  107) are evidence of this warm climate.  Corals require shallow, clear water in which the temperature remains above 18° C (65° F).  Most of the corals alive today are located within 30° north or south of the equator and live at or close to sea level.  Using a uniformitarian line of reasoning, which allows one to examine current conditions in order to speculate about the past, the presence of coral fossils supports the idea that eastern New York state was once part of a shallow sea located close to the equator.

During the Devonian period significant changes occurred in the world’s geography as North America (Laurentia) and Europe collided to form Euramerica between 408 – 387 million years ago.  The super continent of Euramerica along with that of Gondwana would later form the famous giant continent of Pangea about 200 million years later.  The collision with Europe resulted in the formation of new mountains now located over Greenland and Scandinavia (Britannica 2001).

In the Middle Devonian mountains continued to form as the shock of the collision reverberated west.  The Acadian orogeny in the eastern United States resulted in the creation of the Appalachian mountains.  The uplift from the Acadian orogeny triggered the erosion and deposition of large quantities of sediments into the Helderberg shallow sea.  Sedimentation was so great, that by the end of the Middle Devonian a number of deltas formed around the centers of high sediment input (Faill 1997).

Paleoecology
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As terrigenous debris from the Appalachians were deposited from the east into the Helderberg sea, it expanded northward and westward.  LaPorte believed that the formations which make up the Helderberg group were “separate facies deposited contemporaneously within different areas of a westward transgressing sea” (99).  Thus each formation correlates to a section of the sea, and represents different sedimentary environments (Table 1.).  Each of these formations can be ascribed to a different part of the shallow sea using information from the fossils and physical structures found within them.

For example, common fossils in the Manlius formation include algal structures, rugose corals, snails, and clams.  Therefore we can assume that the paleoenvironment was marine with very shallow water and plenty of light, as this is the environment best suited for those organisms.  In addition, mud cracks and erosion surfaces were noted within the Manlius formation, indicating that it was temporarily subaerilly exposed, dried out, and eroded.  Remains of bioherms and vertical burrows were also found in the Manlius.  Taken together, this evidence corresponds well with supratidal and intertidal environments where the water level rises and falls and waves may erode the sediments.

The overall picture from east to west begins with New Scotland; an area of calm marine waters with good circulation, and deep enough so that deposition occurred below wave base.  Terrigenous detritus from farther east entered into the sea in part, forming argillaceous limestone.  Fossils of brachiopods like Leptaena and Eospirifer, along with bryozoans, trilobites and ostracods are found throughout the formation (Bordeaux 1999).

Further west the sea became shallower.  “The strong water agitation in this area removed carbonate mud, current stratified the sediments, and supported an abundant and reasonably diverse group of sessile, suspension-feeding organisms” (LaPorte 1969: 116).  These deposits represent the Coeymans facies, a subtidal, high-energy near shore, inner shelf environment.

West of the Coeymans was a broad lagoon with no waves or currents.  Lack of regular current and wave action, and distance from more open, deeper marine waters, resulted in a restricted quiet water environment.  Carbonate mud built up to intertidal and supratidal levels forming local tidal flats in the Manlius formation.  These tidal flat deposits then migrated across subtidal sediments and formed a complex combination of supratidal, intertidal and subtidal units within Manlius.  (LaPorte 1969:  115-16)

During the deposition of the Becraft and Alsen formations the sea regressed.  The Alsen formation consists of black shales interbedded with “normal marine” strata, indicating an alternation between anoxic and aerobic conditions (LaFleur & Mazzo 1984).  At some point during the lower Devonian, the sea began to transgress again, and this continued through the deposition of the Port Ewen formation.

The black shales of the Port Ewen formation indicate a deep marine depositional environment well off shore.  Port Ewen can be divided into the lower Lake Katrine member and the Upper East Kingston member.  The Lake Katrine formation is largely clay’/mudstone to shale.  The overlying East Kingston formation, nearly twice as thick as the Lake Katrine in some places, consists of clay rich laminate alternating with black chert and calcite laminate with discrete, sometimes cyclical, carbonate zones.  Abundant burrowing and other trace fossils suggest an active infaunal community in this largely anoxic realm.  However, the presence of current-transported fauna and fossil fragments indicate its proximity to the upper slope/shelf region.  (LaFleur & Mazzo 1984)

The Tristates (Oriskany) Group and Onondaga formation were also deposited in New York state during later parts of the Devonian period.  The Tristates group encompasses the Glenerie, Esopus, Carlisle Center, and Schoharie formations.  The earliest, Glenerie, consists largely of silty limestone while the more massive Esopus is silty mudstone and contains Zoophycos trace fossils.  The Carlisle Center and Schoharie formations are a mix of siltstone and siltstone with limestone nodules.  The Onondaga formation, deposited during the Middle Devonian, is composed of largely silty limestone.  The change from primarily limestone to primarily siltstone indicates a change from a marine environment to a rejuvenated stream or river environment (Hussey 1944: 152).

Summary
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New York state was located underneath a large shallow sea during the Late Silurian to Middle Devonian.  North America, then part of the continent of Laurentia, was located very close to the equator.  As a result, New York’s climate was very hot and humid, similar to that of the Bahamas or tropics.  The Late Silurian and Devonian were times of great paleogeographic change.  The Caledonian, Accadian and Appalachian orogenies all occured during this time.  Great mountains were formed as a result of the convergence of Laurentia and Europe.  The newly formed Appalachian mountains to the east of New York state deposited eroded sediment into the shallow sea. 

Meanwhile the formations that make up the Helderberg Group and later Tristates Group and Ondondaga formation were deposited, each representing a different sedimentary and ecologically different environment within the shallow sea.  The fossils and mineral features preserved within them today provide information about the warm marine paleoenvironment that once existed in the Later Silurian-Middle Devonian in New York state.

Today the New York Silurian and Devonian rock formations offer a vivid glimpse of the marine life during these periods.  The well-defined and well-preserved strata and fossils make for an excellent example of a “journey through time” for roadside geologists.  However, it is important to remember that mainly shelled and hard-bodied animals were able to preserve in the sediments.  It is unfortunately a rare occurrence in paleontology to find soft-bodied fossils from the Silurian and Devonian sediments of New York.  One can only hypothesize how divers and well populated these communities actually were with other invertebrates and fish that were not preserved to satisfy our curiosity.

Table 1.  Adapted from La Porte’s and Bordeaux’s Environmental Interpretations of the New York Late Silurian – Middle Devonian Formations.  (top)
 

Bibliography
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Brett, Carlton E., et al.  “Silurian-Early Devonian sequence stratigraphy, cycles and paleoenvironments of the Niagara Peninsula area of Ontario, Canada.”  Geological Society of America, Annual Meeting, Field Trip Guidebook 16 (1998).

Crowell, John C. “Pre-Mesozoic ice ages; their bearing on understanding the climate system.”  Memoir – Geological Society of America.  192 (1999): 106.

Diedrich, Nathaniel W. and Bruce H. Wilkinson.  “Depositional cyclicity in the Lower Devonian Helderberg Group of New York State.”  Journal of Geology.  107, no. 6 (1999): 643-658.

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Faill, Rodger T.  “A geologic history of the north-central Appalachians; Part 2, the Appalachian Basin from the Silurian through the Carboniferous.”  American Journal of Science.  297, no. 7 (1997): 729-761.

Hussey, Russell C.  Historical Geology:  The Geology of North America.  New York:  McGraw-Hill, 1944.

LaFleur, Robert G. and Carl R. Mazzo.  “Stratigraphy of the Port Ewen Formation (Lower Devonian), Eastern New York.”  Northeastern Geology.  6, no. 2 (1984): 71-82.

LaPorte, Leo.  “Recognition of a Transgressive Carbonate Sequence Within an Epeiric Sea: Helderberg Group (Lower Devonian) of New York State.”  Depositional Environments in Carbonate Rocks.  Special publication - Society of Economic Paleontologists and Mineralogists, no. 14.  Ed.  G.M. Friedman.  Tulsa, Oklahoma:  1969.

Scotese, Christopher R.  Paleomap Project.  c. 2001.  < http://www.scotese.com >.

University of California, Berkley.  Museum of Paleontology.  c. 1994-2001.  < http://www.ucmp.berkeley.edu/index.html >.