Abstract Miocene strata exposed in the Calvert Cliffs, along the western shore of the Chesapeake Bay, Maryland, have a long history of study owing to their rich fossil record, including a series of spectacular shell and bone beds. Owing to increasingly refined biostratigraphic age control, these outcrops continue to serve as important references for geological and paleontological analyses. The canonical Calvert, Choptank, and St. Marys Formations, first described by Shattuck (1904), are generally interpreted as shallowing-up, from a fully marine open shelf to a variety of marginal marine, coastal environments. More detailed paleoenvironmental interpretation is challenging, however, owing to pervasive bioturbation, which largely obliterates diagnostic physical sedimentary structures and mixes grain populations; most lithologic contacts, including regional unconformities, are burrowed firmgrounds at the scale of a single outcrop. This field trip will visit a series of classic localities in the Calvert Cliffs to discuss the use of sedimentologic, ichnologic, taphonomic, and faunal evidence to infer environments under these challenging conditions, which are common to Cretaceous and Cenozoic strata throughout the U.S. Gulf and Atlantic Coastal Plains. We will examine all of Shattuck‚s (1904) original lithologic “zones” within the Plum Point Member of the Calvert Formation, the Choptank Formation, and the Little Cove Point Member of the St. Marys Formation, as well as view the channelized “upland gravel” that are probably the estuarine and fluvial equivalents of the marine upper Miocene Eastover Formation in Virginia. The physical stratigraphic discussion will focus on the most controversial intervals within the succession, namely the unconformities that define the bases of the Choptank and St. Marys Formations, where misunderstanding would mislead historical analysis.

Cores recovered on Deep Sea Drilling Program leg 43 and on Bermuda itself, together with geophysical data (anomalies in basement depth, geoid, and heatflow) and modeling have long suggested that the uplift forming the Bermuda Rise, as well as the initial igneous activity that produced the Bermuda volcanoes, began ca. 47–40 Ma, during the early to middle part of the Middle Eocene. Some authors attribute 65 Ma igneous activity in Mississippi and 115 Ma activity in Kansas to a putative “Bermuda hotspot” or plume fixed in the mantle below a moving North America plate. While this is more or less consistent with hotspot traces computed from “absolute motion” models, the hotspot or plume must resemble a blob in a lava lamp that is turned off for up to 25 million years at a time, and/or be heavily influenced by lithosphere structure. Moreover, Cretaceous igneous activity in Texas and Eocene intrusions in Virginia then require separate mantle “blobs.” The pillow lavas forming the original Bermuda shield volcano have not been reliably dated, and the three associated smaller edifices have not been drilled or dated. A well-dated (ca. 33–34 Ma) episode of unusually titaniferous sheet intrusion in the Bermuda edifice was either triggered by platewide stress changes or reflects local volcanogenic events deep in the mantle source region. The high Ti and Fe of the Bermuda intrusive sheets probably relate to the very high-amplitude magnetic anomalies discovered on the islands. Numerical models constrained by available geophysical data attribute the Bermuda Rise to some combination of lithospheric reheating and dynamic uplift. While the relative contributions of these two processes cannot yet be wholly separated, three features of the rise clearly distinguish it from the Hawaiian swell: (1) the Bermuda Rise is elongated at right angles to the direction of plate motion; (2) there has been little or no subsidence of the rise and the volcanic edifice since its formation—in fact, rise uplift continued at the same site from the late Middle Eocene into the Miocene; and (3) the Bermuda Rise lacks a clear, age-progressive chain. We infer that the Bermuda Rise and other Atlantic midplate rises are supported by anomalous asthenosphere, upwelling or not, that penetrates the thermal boundary layer and travels with the overlying plate. The elongation along crustal isochrons of both the Bermuda volcanoes and the Bermuda Rise and rise development mostly within a belt of rougher, thinner crust and seismically “slower” upper mantle—implying retention of gabbroic melts at the ancient Mid-Atlantic Ridge axis—suggest that the mantle lithosphere may have helped localize rise development, in contradiction to plume models. The Bermuda Rise area is seismically more active than its oceanic surroundings, preferentially along old transform traces, possibly reflecting a weaker upper mantle lithosphere. We attribute the “Bermuda event” to a global plate kinematic reorganization triggered by the closing of the Tethys and/or the associated gravitational collapse into the lower mantle of subducted slabs that had been temporarily stagnant near the 660 km mantle discontinuity. The widespread onset of sinking slabs required simultaneous up-welling for mass balance. In addition, the global plate kinematic reorganization was accompanied by increased stress in some plate interiors, favoring magma ascent along fractures at structurally weak sites. We suggest that the Bermuda event and concomitant igneous activity in Virginia, West Antarctica, Africa, and other regions were among such upwellings, but structurally influenced by the lithosphere, and probably originated in the upper mantle. Drilling a transect of boreholes across and along the Bermuda Rise to elucidate turbidite offlap during rise formation might discriminate between a widely distributed mantle source (such as a previously subducted slab) and a narrow plume whose head (or melt root) spreads out quasi-radially over time, generating an upward and outward expanding swell.

The origin of off–Mid-Oceanic-Ridge (MOR)-axis paired (conjugate) basement ridges and other conjugate structures is examined, with a focus on the North Atlantic. Paired-ridge morphologies are found at volcanic edifice scales (influencing ∼25- to 75-km spreading boundary) and at 200- to 1000-km scales (where structures may be V-shaped, suggesting propagation along the axis at rates from a few to 200 mm/yr). Both scales modulate the longest-scale along-axis MOR topographic anomalies (∼3100 km for the Azores and ∼3800 km for Iceland). At the short scale, MOR axial magma centers with off-axis “split-volcano” pairs suggest magmatic episodicity at 0.1- to 1-m.y. intervals, erratic along-strike displacements (1–10 km) between episodes, and no fixity in a hotspot frame. However, along-strike axis motion (calculated from the Gripp and Gordon, 2002, model) seems to inhibit formation of organized, long-lived axial volcanoes. Intermediate-scale ridge pairs have been attributed to “blobs” and temporal variability of mantle plumes, but some may have formed by passive tapping of anomalous mantle patches as the MOR migrates across them. A great variety of such passive “conjugate ridges” is geometrically possible as a function of MOR velocity over the mantle, the spreading rate and its asymmetry, the plan-view shape of the MOR axis (with transforms, normal, and oblique spreading), and the mantle source, whose shape (outline) could be constrained if the model is shown correct. Among observed ridge pairs (e.g., Morris Jesup Rise/Yermak Plateau; Reykjanes Ridge V-shaped ridges; East and West Thulean Rise; Flores and Faial Ridges; J-Anomaly and Madeira ridges; Ceara and Sierra Leone ridges), some have sharp older and/or younger edges, implying that the anomalous mantle sources, whether fixed or not, also have sharp boundaries, some of which are diachronous, implying along-strike propagation. The short and long scales appear nearly symmetrical along-strike, but the intermediate scales, including geochemical and isotope anomalies in axial basalts, suggest preferential southward propagation, perhaps reflecting the southward astheno-sphere motion predicted by counterflow models. Discrimination between the “passive heterogeneity” and “active plume” concepts is discussed.

Abstract In creating Volume M, the Western North Atlantic region (Vogt and Tucholke, 1986a) for the Geology of North America series, we deemed it best from both oceanographic and plate-tectonic viewpoints to deal with the entire North Atlantic spreading system from the equator to the Arctic (Figs. 1 and 2), rather than limiting treatment to the western half of the ocean basin. Even so, the scope in some places had to be expanded. The Atlantic, like other ocean basins, did not evolve in isolation from global changes in tectonic regime, oceanic circulation, or climate patterns (Fig. 3). The development of plate-tectonic theory since the late 1960s clearly has emphasized the importance of these large-scale linkages. The present chapter continues this philosophy, summarizing the geology of the North Atlantic but noting linkages to areas outside this ocean basin. The synthesis is based largely on material presented in Volume M. The citation or lack of citation of Volume Μ references here, however, reflects only the thematic fabric of the present synthesis, not the scientific merit of the chapters. We refer the reader to original sources in Volume Μ for more complete treatment. We begin this chapter by noting ties between Volume Μ and several other Geology of North America volumes, and we continue with some “vital statistics” that describe three basic components of the Atlantic in space and time: igneous crust, sediments, and ocean waters. This is followed by a discussion of scales of spatial and temporal variability, with emphasis on the latter. The chapter concludes with a summary of some of the important advances that have occurred in the three years since Volume Μ was published.

Location The Cliffs of Calvert (Calvert Cliffs) form a series of wavecut bluffs up to 100 to 130 ft (30 to 40 m) high, which extend about 25 mi (40 km) along the western shore of the Chesapeake Bay (Fig. 1). For brevity, we refer to sites by distance in miles (km) either + (north) or - (south) of Governors Run, and keyed to Figure 2. Maryland 2/4 is the main route of access and runs parallel to the cliffs. Most of the Calvert Cliffs have been residentially developed so public access from the land is limited to commercial marinas (CM), public parking (PP), and simple road access (RA; Fig. 1). There are two parks along the cliffs: the Flag Ponds Natural Area and the Calvert Cliffs State Park. The cliffs behind Flag Ponds lack good exposures: those at the Calvert Cliffs State Park can be reached only by a 0.25 mi (0.4 km) footpath. Permission must be obtained for access through private property. Digging into or climbing on the cliffs is dangerous and prohibited. Significant fossil finds such as vertebrate remains should be reported to the Calvert Marine Museum. The best geologic overview of the cliffs is by boat, with landings at selected sites (public beach access in Maryland extends up to Mean High Water).

Abstract This volume deals with the geology and geophysics of the western North Atlantic, the basin between the Mid-Atlantic Ridge and the eastern margin of North America. Although the book’s focus is on the “North American Basin” (Fig. 1), individual chapters and charts extend the bounds beyond the “western North Atlantic.” These extensions were dictated by the geology, which cannot be synthesized meaningfully if constrained by artificial geographic or political boundaries. Generally speaking, the content of this volume is contained within the region surrounded by the continental margins of the Antilles, eastern North America, Greenland, and Iceland in the west and north, the Mid- Atlantic Ridge crest on the east, and approximately 10°–15° N latitude to the south. However, in dealing with the Mid-Atlantic Ridge, plate kinematic models, and Atlantic paleogeography, we have extended our scope north into the Arctic region and east into the eastern Atlantic in order to present a complete synthesis of this major plate boundary and its evolution through time. The significance of Iceland for oceanic geoscience often has been overlooked or underrated; to help remedy this, two chapters deal mainly with this remarkable subaerial exposure of the Mid-Atlantic Ridge. One topic, mid-plate seismicity and stress, also demanded a plate-wide treatment covering the entire “stable” lithosphere from the Mid-Atlantic Ridge to the western Cordillera. Placing this treatment in the North Atlantic synthesis presumes that mid-plate stress is closely related to absolute plate motion (which is calculated largely from oceanic data) and/or to “ridge-push” forces from the

Abstract Bathymetry is the science and technology of ocean depth measurement. In the narrow and traditional sense, the objective of bathymetry is to define the seafloor topographic surface, h(x, y). Most bathymetric studies also interpret the depth data— usually displayed in contour charts or profiles—in terms of undersea geomorphology and, in concert with other geophysical data, as a constraint on sub-seafloor geologic structure. In many parts of the ocean, bathymetry is about the only type of geological information available. Thus, compared to land, seafloor topography necessarily plays a larger role in geologic interpretations. The North Atlantic Ocean covers an enormous area (4 × 10 7 km 2 ) with diverse and numerous geomorphic features at many scales. Bathymetric control is very uneven in distribution and quality. Some areas have been surveyed by overlapping swaths of multibeam bathymetry with reliable mapping of features less than 1 km apart, while other areas of 10 3 or 10 4 km 2 have no soundings, particularly in the equatorial latitudes. Unlike many previous studies, this chapter does not offer a systematic guided tour of the submarine topography. (See Holcombe, 1977 for a glossary of ocean-bottom land forms.) Recent syntheses of Atlantic bathymetry include Vogt and Perry (1982) and Emery and Uchupi (1984). Instead, the potential of seafloor “imaging” is evaluated over a large range of spatial scales, approximately ten orders of magnitude from the form of the entire Atlantic basin down to the size of grains in seafloor sediments (Fig. 1). Under “submarine imaging” are included 3-D (stereoscopic) descriptions of bottom

Abstract We present an integrated geomagnetic polarity and geologic time scale for the Jurassic to Recent interval, encompassing the age range of the modern ocean floor. The time scale is based on the most recent bio-, magneto-, and radiochronologic data available. The biostratigraphic bases for Jurassic, Cretaceous, and Cenozoic time-scales are discussed extensively elsewhere (e.g., Gradstein, this volume; Van Hinte, 1976b; Hardenbol and Berggren, 1978; Berggren and Van Couvering, 1974; Van Couvering and Berggren, 1977). Emphasis is placed here on magnetochronology and its integration with biochronology in the derivation of an internally consistent geologic time scale. The binary signal of normal and reversed geomagnetic polarity has little intrinsic absolute time value (ordinal scale), but it can be used to measure time according to its radiochronologic calibration (cardinal scale). The standard magnetic reversal sequence has a correlatable, characteristic pattern and is demonstrated to be continuous from numerous marine magnetic anomaly profiles from the world ocean. The reversal sequence is recorded by lateral accretion in sea-floor spreading and vertical accumulation in sedimentary or lava sections, allowing independent checks on the completeness and relative spacing of the reversal sequence as well as the opportunity to apply an assortment of geochronologic data for calibration. Although different phenomena and assumptions are invoked in their derivation, both magneto- and bio- chronologic time estimates involve indirect assessment according to calculated rates of sea-floor spreading, sedimentation, and biotic evolution. These extend the application of the relatively few reliable radiometric dates available, so that a continuous

Abstract By 1854, there were enough deep ocean soundings to allow M. F. Maury to draw the first bathymetric chart of the North Atlantic Ocean in 1855. He defined the great shoaling “middle ground” of the Atlantic basin, later to be known as the Mid-Atlantic Ridge (MAR). It is interesting to note that during the Challenger expedition (1872–1876), Sir Wyville Thomson predicted, on the basis of water temperatures alone, that the MAR is a nearly continuous topographic barrier dividing the Atlantic Ocean. In the 1950s, continuous echogram profiles revealed the presence of a deep median valley along the axis of the MAR (Heezen and others, 1959; Hill 1960). Seismicity (Gutenburg and Richter, 1954) and the occurrence of youthful basaltic lavas on the MAR (Shand, 1949) indicated that the median valley was a geologically active rift zone. Heezen (1960) first suggested that extension across the rift valley might be responsible for accretion of basaltic oceanic crust and the drift of the continents, but his contention that this rifting resulted from expansion of the earth was incorrect. High heat flow measured on the MAR (Bullard and Day, 1961) and large axial magnetic anomalies (Heezen and others, 1959) were added to seismicity and recent volcanism as enigmas associated with the rift valley. In his seafloor spreading hypothesis Hess (1962) elegantly explained these puzzling observations in a unified model, and the bilateral symmetry of magnetic anomalies found on other midocean ridges (Vine and Matthews, 1963; Vine, 1966)

Abstract Three regions of subaerial volcanism occur near the eastern edge of the North America plate: Iceland, Jan Mayen and the Azores. In Iceland the ridge axis emerges above sea level, most of the volcanic pile was generated subaerially along its trace, and superimposed flank zones are situated off the accretion zone. The Azores lie on both sides of the oceanic ridge axis topping an extensive platform adjoining it. The platform was generated by excess volcanic production on the transecting ridge crest similar to the ridge formation in Iceland. The islands are stratovolcanoes formed off the ridge crest by flank volcanism. The volcanism and tectonic setting of Jan Mayen and the Azores is compared to the flank zones of Iceland, of which they appear to be close analogs. Direct access to an active oceanic spreading axis makes Iceland particularly significant. The crust generated there suffers erosion which exposes the uppermost 1–2 km for three dimensional view. Subaerial volcanism in Iceland’s accretion zone produces a layered igneous sequence due to a wider rift zone and extensive lateral spread of lava flows as opposed to the deep ocean ridge with a much narrower rift and outflow zone of lava. Subaerial spreading may also have been important in early rifting of the North Atlantic margins (Hinz, 1981). As a result, submerged layered extrusives with units thickening downdip towards the spreading axis and offlapping in the direction of spreading may exist near some of the North Atlantic continental margins (Tucholke and Ludwig, 1982).

Abstract During the early 1960s, when the idea of seafloor spreading was crystallizing out of geophysical data gathered over oceanic areas, similar ideas were beginning to take shape in Icelandic geology, but on the basis of a different kind of data. Geological studies of the structure of the eastern Iceland Tertiary lava pile led Walker (1959, 1960) to suggest that regional dips of the lavas were due to sagging accompanying lava deposition in the active volcanic zone. These ideas were further elaborated by Bodvarsson and Walker (1964), who tried to estimate the amount of crustal drift that might have accompanied the injection of dikes as observed in the eastern Iceland lava pile, taking into account the increase in dike fraction with depth in the crust. Subsequent studies of the Tertiary lava pile in other parts of Iceland have essentially confirmed a structure analogous to that given by Walker for eastern Iceland (Saemundsson, 1978). Observations from two distinct geological regions can be brought to bear on the process of crustal accretion in Iceland. One region is the present active zone of rifting and volcanism, the axial rift zone. The other region is the flanking Tertiary lava pile where the internal structure of the uppermost well-developed crust can be observed to a depth of 1–1.5 km in valleys that have been carved into the crust by glacier action. In the axial rift zones the constructional processes of extrusive volcanism, faulting, and Assuring take place as discrete but closely associated events.