104 NORTH AMERICA/Atlantic Margin between shallow (2000 m) and deep (more than 4000 m) During the Cenozoic, continental-margin sedimentation was increasingly affected by currents, especially contour-parallel currents (see Sedimentary Environments: Contourites) Depocentres shifted seawards, causing the continental rise to be built up (Figure 12) The Paleocene and Eocene probably saw the onset of a current-dominated regime (e.g the ancestral Gulf Stream) and the initiation of small canyons, but much of this early record is thin or missing because of subsequent erosion during the Oligocene Global temperatures began falling as the Earth completed its transition from a ‘greenhouse’ in the Cretaceous and Paleocene to an ‘icehouse’ in the Oligocene During the Middle Oligocene sea-level lowstand, the Baltimore Canyon shelf retreated by about 30 km Cold water in the North Atlantic contributed to deep thermohaline circulation, and contour currents became more vigorous in the Early Miocene when cold Norwegian bottom waters overflowed the Norwegian–Greenland ridge and entered the Atlantic, initiating the formation of both the elongate Chesapeake and the Blake Outer Ridge drift deposits Glaciation of North America in the Late Tertiary increased siliciclastic deposition, intensified down-slope sedimentary processes (e.g turbidity currents, slumps, slides, and debris flows), and Figure 12 The bathymetric positions of the main depocentres in the Baltimore Canyon trough through the last 187 Ma Large dots indicate major depocentres (i.e thickest sediments) Small dots indicate secondary depocentres Solid line shows the shift in the primary deep water depocentres through time Dashed line shows the shift in the primary shallow water depocentres This chart shows the migration of the trough seawards and landwards during the history of the margin Note the marked period of limited shelf and slope deposition in the Late Miocene (Reproduced from Poag CW and Sevon WD (1989) A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the US Middle Atlantic Continental Margin Geomorphology 2: 119 157.)