8. Terrain and the Battle of Fredericksburg, December 13, 1862
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3. The Paleozoic Record of Changes in Global Climate and Sea Level: Central Appalachian Basin By C. Blaine Cecil,1 David K. Brezinski,2 and Frank Dulong1 With stop contributions by Bascombe Blake,3 Cortland Eble,4 Nick Fedorko,3 William Grady,3 John Repetski,1 Viktoras Skema,5 and Robert Stamm1 Introduction to the Field Trip ally attributed to trapping of sediment in estuaries in response to sea-level rise. More specifically, basal sandstone units of the Cambrian, Silurian, Mississippian, and Pennsylvanian in By C. Blaine Cecil the Appalachian basin are generally attributed to influx induced by tectonic uplift, whereas laterally extensive shale This field trip provides an update of an earlier trip that deposits, such as the Ordovician Martinsburg Formation and was conducted in conjunction with the 1998 Annual Meeting Devonian black shale units, are inferred to result from sedi- of The Geological Society of America (Cecil and others, ment trapping or autocyclic deposition of prodelta muds. 1998). The present trip emphasizes global climate change and Although tectonics clearly plays a role in uplift and subsi- eustatic sea-level change (allocyclic processes of Beerbower, dence, and eustasy also governs the amount of accommoda- 1964) (in a global paleogeographic context) as the predomi- tion space, variation in climate, particularly long- to short- nant controls on Paleozoic sedimentation and stratigraphy in term variation in rainfall in tropical conditions, is of greater the Appalachian foreland basin. The trip stops illustrate long- importance as a control on sediment supply and sedimenta- to short-term paleoclimate change (table 1) as the predomi- tion (for example, Ziegler and others, 1987; Cecil, 1990 and nant control on sediment supply, both chemical and siliciclas- references therein; Cecil and others 1993; Cecil and others, tic (fig. 1A, B). Interpretations of climate variations are based 2003a). The trip stops illustrate why climate change is a far on paleoclimate indicators such as paleosols, physical and more important first-order control on stratigraphy than is gen- chemical sedimentation, sedimentary geochemistry and min- erally recognized. eralogy, and paleontology. Selected stops in Paleozoic strata provide evidence for Classification of Tropical Rainfall sedimentary response to paleo-tropical rainfall regimes that ranged in duration from long term to short term (table 1) and Many definitions of tropical climatic regimes are based in rainfall amounts that ranged from arid to perhumid (table on annual rainfall; few, however, attempt to incorporate sea- 2, fig. 1F). Eustatic processes also are discussed at each stop, sonality of rainfall (for example, Thornthwaite, 1948). It is and the role of tectonics may be mentioned briefly where becoming increasingly apparent, however, that it is the sea- appropriate. These later two allocyclic processes are generally sonality of annual rainfall that governs weathering, pedogene- accepted as the predominant control on sediment supply in sis, variations in soil moisture, vegetative cover, and erosion contrast to climate, which is often not considered. For exam- and sediment yield for a given catchment basin (for example, ple, tectonic uplift is commonly considered as the predomi- Ziegler and others, 1987; Cecil, 1990, 2003; and Cecil and nant control on rapid influxes of siliciclastic material into others, 2003a and references therein), not the total amount of depocenters, whereas siliciclastic sediment starvation is usu- annual rainfall. In order to assign degrees of seasonality, cli- mate regimes, as used herein, are based on the number of wet months in a year. A wet month is defined as a month in 1U.S. Geological Survey, Reston, VA 20192. which precipitation exceeds evapotranspiration (table 2, fig. 2 Maryland Geological Survey, Baltimore, MD 21218. 1F) (Cecil, 2003). 3West Virginia Geological and Economic Survey, Morgantown, WV 26507. By the limits set forth in table 2 and figure 1F, both arid 4Kentucky Geological Survey, Lexington, KY 40506. and perhumid conditions are nonseasonal. All other rainfall 5Pennsylvania Geological Survey, Middletown, PA 17057. conditions have some degree of seasonality. Maximum sea- 78 Geology of the National Capital Region—Field Trip Guidebook Figure 1. A, Potential for fluvial siliciclastic load as function of climate; B, Potential for fluvial dissolved load as a function of climate; C, Potential for peat formation as a function of climate; D, Potential for eolian transport as a function of climate; E, Formation of U.S. Department of Agriculture soil orders as a function of climate; and F, Climate classification based on the number of wet months per year. From Cecil and Dulong (2003). sonality and maximum fluvial siliciclastic sediment supply both dissolved and siliciclastic sediment supply approach zero occur under dry subhumid conditions when there are approxi- (Cecil and others, 1993). The conditions for eolian transport mately three to five consecutive wet months (fig. 1A). Maxi- in sand seas are illustrated in figure 1D. mum dissolved inorganic supply occurs under semiarid to dry Climate is also a major control on weathering and soil subhumid conditions (fig. 1B). The most ideal conditions for formation (pedogenesis); conversely, paleosols can be used to the formation and preservation of peat, as a precursor to coal, reconstruct paleoclimate and paleo water-tables. Climate may occur under humid and perhumid conditions (fig. 1C) when be the primary control on pedogenesis in epicontinental The Paleozoic Record of Changes in Global Climate and Sea Level: Central Appalachian Basin 79 Table 1. Tropical and subtropical climate change classification the movement of the continents through paleolatitudes. The (modified from Cecil, 1990, 2003). region of what is now the central Appalachian basin moved northward from about lat 40º S. in the latest Precambrian and Relative duration Cause Time (years) Early Cambrian to lat 30º S. ± 5º during the Early Ordovician where it remained well into the Mississippian (Scotese, 6 8 Long term Continental drift; 10 –10 1998). Northward movement continued from about lat 30º S. orogenesis; 105–107 in the Early Mississippian to about lat 3º N. by the beginning Ice house/ greenhouse of the Permian (figs. 2–9). From the perspective of zonal atmospheric circulation, the field trip study area moved from Intermediate term 100- and 400-ka cycles 105 the dry subhumid belt of the southern hemisphere (prevailing of orbital eccentricity; easterlies) in the Early Cambrian into the high pressure belt glacial/interglacial of aridity by Late Cambrian where it remained well into the Mississippian. Late Precambrian and Early Cambrian sedi- Short term Cycles in orbital 104 ments are dominated by siliciclastics, whereas Middle obliquity and Cambrian through Early Devonian strata contain abundant precession limestone, dolomite, and evaporites. These strata are totally consistent with paleogeographic interpretations (for example, Very short term Uncertain 103 Scotese, 1998). By the Late Devonian the region began to (Millennial) move northward toward the humid low-pressure equatorial Instantaneous Weather 10-2 region. Movement continued through the equatorial region (weeks, days, or during the Pennsylvanian. hours) Orbital Climate Forcing Table 2. Tropical climate regimes and degree of seasonality There are clear and distinct effects of orbital parameters based on the number of consecutive wet months per year (modi- (obliquity, eccentricity, and precession) on intermediate- to fied from Thornthwaite, 1948, and Cecil, 2003). short-term climate change. Orbital climate forcing is an important control on variation in sediment supply because Degree orbital parameters have a marked effect on the seasonality of Average number Climate of incoming solar radiation (insolation), which results in varia- of wet months regime seasonality tions in the seasonality of rainfall. As a result, orbital climate forcing plays a major role in sedimentation and stratigraphy. 0 Arid Nonseasonal The lithostratigraphic signal induced by intermediate- to 1–2 Semiarid Minimal short-term climate change may be somewhat suppressed dur- 3–5 Dry subhumid Maximum ing periods of long-term aridity or humidity, but is not oblit- 6–8 Moist subhumid Medial erated as is illustrated at stops in Ordovician, Silurian, and 9–11 Humid Minimal 12 Perhumid Nonseasonal Pennsylvanian strata. basins where other parameters, such as parent material and Tectonics relief, are relatively constant. On the basis of structure, chem- istry, and mineralogy, paleosols can be classified at the level As pointed out above, continental drift may explain of soil orders using the U.S. Department of Agriculture classi- some of the long-term climate changes in the Appalachian fication (Soil Survey Staff, 1975). Once classified, paleosols basin, but it does not explain all such changes. Some long- can be used to interpret paleoclimate (fig. 1E), including the term climate changes may be better explained by the alter- amount and seasonality of rainfall as is illustrated at a number ation of atmospheric zonal circulation as a result of mountain of stops on the trip. building. Such effects are well known, as illustrated by the development of the Asian monsoon in response to the forma- Mechanisms of Climate Change tion of the Himalayan Mountains and the Tibetan Plateau, when the Indian subcontinent collided with Asia. Similar tec- tonic controls on late Paleozoic paleoclimate have been sug- Continental Drift