Allostratigraphy and formalizing the Cold Harbor Alloformation, Virginia Coastal Plain, Eastern Virginia
Introduction Distinct morphologic features in the Virginia part of the Coastal Plain Province (herein referred to as the Virginia Coastal Plain) were noted in one of the earliest geologic surveys in 1839 (Rogers, 1884). South of the James River, Rogers (1884, p. 252) described a western bench that ranged from “a low rocky coast…of the primeval sea” to an eastern facing scarp east of the City of Suffolk, Virginia. His eastern bench extended from the scarp at Suffolk, which he suggested was a former shoreline, to the present Atlantic Ocean. Notably, the eastern plain was described as less dissected than the western plain. In Virginia, creation and publication of topographic maps by the U. S. Geological Survey began in the mid-1880s. By 1905, many areas in eastern Virginia were mapped as 30-minute and 15-minute series with 10-foot (ft) contour interval accuracy (Virginia Division of Mineral Resources [VDMR] , 2008). After the continued release of these early maps that displayed Virginia Coastal Plain morphology in increasingly greater detail, several geological reports were published where scarps and plains were traced, named, and origins proposed (Shaler, 1890; Shattuck, 1901; Clark and Miller, 1906, 1912; Wentworth, 1930). Shattuck (1901) recognized three continuous scarps in Maryland and most importantly suggested that there was a correlation between surficial deposits (geologic formations) and morphologic terraces; this defined the terrace-formation concept that became the basis of geologic mapping of the Upper Pliocene and Pleistocene sediments in the Virginia Coastal Plain. Clark and Miller (1912) described three Pleistocene formations and supported the terrace-formation concept because the sediments within each formation were lithologically similar to one another, and differentiating them by comparing key vertical sections was not helpful. In Virginia, Wentworth (1930) defined and mapped five Pleistocene terraces on the basis of correlating morphologic terraces with their underlying formation sediments. Similarly, Cooke (1931) identified and traced seven marine shorelines and proposed corresponding terrace names from Maryland to Florida. Cooke (1931) also pointed out that the various elevations of scarps were related to interglacial sea-level highstands whereas prior workers called on vertical land movement of emergence and submergence to create shoreline scarps. Cooke’s (1936) geologic map of eastern South Carolina showing the Pleistocene sediments is also based on the terrace-formation concept. Detailed geologic mapping of the Virginia Coastal Plain by Virginia Division of Mineral Resources (VDMR) and U.S. Geological Survey (USGS) followed lithostratigraphic convention for Early Pliocene and older marine sediments. However, geologic mapping of Upper Pliocene and Pleistocene sediments used the concept of morphologic setting and elevations of scarp toe and flat-plain, or the terrace-formation concept. The 1983 version of the North American Stratigraphic Code (hereafter the Code) (North American Commission on Stratigraphic Nomenclature [NACSN] , 1983) introduced allostratigraphy that gave credence to the terrace-formation concept. According to the current Code (NACSN, 2021; Article 58), “an allostratigraphic unit is a mappable body of rock that is defined and identified on the basis of its bounding discontinuities,” and, “formal allostratigraphic units may be defined or distinguish between different (1) superposed discontinuity-bounded deposits of similar lithology, (2) contiguous discontinuity-bounded deposits of similar lithology, or (3) geographically separated discontinuity-bounded units of similar lithology…The lithology of an allostratigraphic unit plays no part in its definition.” However, definition and placement of bounding discontinuities relies on lateral and vertical knowledge of lithology and is so recognized in Article 58e, “A type locality and type area must be designated; a composite stratotype or a type section and several reference sections are desirable.” We understand that each of the terrace formations of Late Pliocene and Pleistocene age were deposited during a marine transgressive event tied to an interglacial sea-level highstand (scarp above the Poquoson Member of the Tabb Formation may be the result of a still-stand as sea level was falling after deposition of the Lynnhaven Member, refer to fig. 1). The base of each map unit is an unconformity (or is at least in part for the base of the Poquoson) and nomenclature for each unit might be changed from “formation” to “alloformation,” as suggested by Doar and Berquist (2009). Figure 1 shows the relative placement of these mapped and named alloformations as used in the Virginia Coastal Plain and Fall Zone (a zone of transition between the Piedmont and Coastal Plain Provinces) (Berquist, 2021). Except perhaps for the Poquoson, many of these proposed alloformations are unlikely to interfinger or grade laterally seaward into extensive deep marine sediments. After their deposition, erosion during sea-level fall and then rise for the next depositional event would seemingly obliterate any marine connection without a basal unconformity. All these alloformations contain sediments deposited in fluvial, estuarine, and shallow-marine environments and are bound everywhere by unconformities. This paper documents the Cold Harbor Alloformation as a formal stratigraphic unit and the rationale for mapping the sediments using an allostratigraphic classification. The Cold Harbor Alloformation is composed of sediments from a variety of environments such as gravel beds, shoreline beach sands, marsh and estuarine sand and mud, and shallow marine sands. These sediments are found at elevations up to 240 ft and lie unconformably above older sediments and rocks. Our work relied on observations of outcrops and many auger borings to identify basal unconformities.