An introduction to the Devonian Period, and the Devonian in New York State and North America

The Devonian strata in New York State were the standard section for North America for over 100 years, and remain a significant reference for regional to global correlation and research. Since publication of L. V. Rickard’s (1975) New York Devonian correlation chart, various higher-resolution stratigraphic analyses have been employed, sometimes at bed-by-bed scale. These include sequence-, bio-, event-, chemo-, and other -stratigraphic approaches, along with increasingly finer-resolution geochronologic dating of airfall volcanic tephras. Results have led to many new interpretations and insights of the succession. The purpose of this three-volume work is to produce a new Devonian stratigraphic synthesis for New York State, and to record, often in detail, current knowledge of the succession, and various other geologic and paleontologic aspects of it for current and future research and discussion. The purpose of this chapter is to provide overviews of the Devonian Period, the Devonian of North America (“Laurentia”), the Devonian of eastern Laurentia, and the Devonian of New York State. Furthermore, this review extends beyond the sedimentary rock and paleobiological record, and beyond the United States, Canada, and northern Mexico, to also summarize aspects of Devonian orogenesis, metasedimentary foreland basin fill, silicic igneous activity, complexities of terranes of Mexico and Central America, and Appalachian faunas that extended into South America. The Devonian Period as a whole encompasses 60 million years of time, approximately 419 to 359 million years ago. During that time, shallow seas covered large continental areas; climate was warmer globally than our current climate, during the late stage of a global greenhouse climate. By the end of the Devonian, that warm climate was descending into a time of global icehouse conditions, with widespread glaciation. The positions of modern continental masses were much different. During the Devonian Period, Life first fully colonized the land, led by primitive spore-bearing plants, small arthropods, and apparently by the Middle Devonian, the first tetrapod (“four-legged”) animals, which evolved from bony fishes. Decimeter-tall plants at the beginning of the period had evolved to tree-size forms by the Middle Devonian, approximately 30 million years later, and Earth’s first forest ecosystems arose. Devonian strata are widespread around the ancient continent “Laurentia,” which approximately corresponds to modern North America). At that time, Laurentia straddled the equator, with New York State and the Appalachian region somewhat north of 30° south latitude. Shallow epicontinental seas covered large but varying amounts of the continent over the period. Mountain belts formed on the eastern, northern, and western margins of Laurentia, due to plate tectonic collisions with smaller continental masses, exotic terranes, and volcanic island arcs. Through the Early to Middle Devonian, seas in western and eastern Laurentia were separated by a “transcontinental arch,” and generally had distinctly different marine faunas. In the latest Middle Devonian, sea level transgressed over the land barrier of the Laurentian Transcontinental Arch and the Canadian Shield, and those marine faunas mixed, leading to a more global cosmopolitan fauna in the Late Devonian. Anomalously, however, Early and Middle Devonian Laurentian shallow marine faunas are found in Devonian rocks in Central and South America, which were part of the southern Gondwana continent, generally thought to be separated from Laurentia by oceanic water depths at that time. During the Devonian, eastern Laurentia was an active tectonic margin, related to continent-continent collisions with various terranes/smaller continental masses. The Caledonian, Acadian, and Neoacadian orogenies resulted in compressional and some transpressional tectonics, and the uplift of an extensive mountain belt from east Greenland to Alabama and Georgia. Crustal loading of the orogen in eastern Laurentia led to subsidence and formation of a retroarc Acadian-Neoacadian Foreland Basin, which was initially filled with marine waters, followed by gradual overfilling to above sea level by massive volumes of synorogenic sediments from the east. The resulting lands were the site of some of the earliest forests on Earth, preserved at several sites in New York State, and forest ecosystems. Large-scale deformation, seismic activity, and metamorphism in the mountain belt were accompanied by igneous processes, including explosive eruption of felsic volcanic ash and other material, collectively termed “tephra,” also sometimes termed ash or tuff layers, or if diagenetically altered, sometimes termed bentonite, K-bentonite, metabentonite, or tonstein layers. These explosive Devonian eruptions sent volcanic tephra high into the atmosphere, and easterly winds spread airfall volcanic “tephra layers” across the eastern United States. Meanwhile, rock decay in the mountains led to the erosion, transport, and deposition of massive volumes of clays, silt, sand, and gravel into the Acadian-Neoacadian Foreland Basin, and beyond. Devonian rocks in New York are found at or just below the surface across approximately 40% of the state (~50,500 km2/19,500 mi2). The strata are generally undeformed and gently dipping, and while often covered by soil, glacial sediments, and vegetative cover, are relatively widely found in natural and man-made exposures. Three relatively thin intervals of carbonates are accompanied by eastward thickening wedges of synorogenic mudrocks, sandstones, and minor conglomerates. The history of geological and paleontological observation and study in New York began in the late 18th century. The first professional geologists appeared in the early 19th century. Since the advent of the first geological survey of New York State in 1836, the Devonian Period (nearly termed the “Erian Period” for New York’s Devonian-age rocks) has been the focus of a great volume of research which continues today. The Devonian succession in New York includes strata from all seven stages of the period, with erosional gaps of small to major significance. In addition to a range of marine facies, nearly one quarter of the entire area of Devonian bedrock in the state was deposited in terrestrial settings, with massive volumes of siliciclastic sediments shed off of Acadian-Neoacadian highlands to the east, that also feature the fossils of Earth’s oldest known forest ecosystems. The stratigraphic philosophy in New York has long evolved toward a hybrid classification, wherein groups, formations, and bed-level units are largely time-rock/allostratigraphic to occasionally chronostratigraphic, with lithostratigraphy often ascribed to member-level divisions (e.g., Pragian to Givetian strata, middle Lower to upper Middle Devonian). However, in some intervals, such as Frasnian strata (lower Upper Devonian), group-level units are time-rock units, and formation-level units within groups are largely lithostratigraphic. Forty-eight years of research since Rickard’s (1975) New York Devonian correlation chart permits development of a new, more refined chart (forthcoming), and also permits a new synthesis of Devonian rocks and fossils in New York, presented in this work of twelve chapters, with additional digital appendices.

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