Abstract The Boyongan and Bayugo porphyry copper-gold deposits are part of an emerging belt of intrusion-centered gold-rich deposits in the Surigao district of northeast Mindanao, Philippines. Exhumation and weathering of these Late Pliocene-age deposits has led to the development of the world's deepest known porphyry oxidation profile at Boyongan (600 m), and yet only a modest (30–70 m) oxidation profile at adjacent Bayugo. Debris flows, volcanic rocks, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the deposits and preserved the supergene profiles. At Boyongan and Bayugo, there is a vertical transition from shallower supergene copper oxide minerals (malachite + azurite + cuprite) to deeper sulfide-stable assemblages (chalcocite ± hypogene sulfides). This transition provides a time-integrated proxy for the position of the water table at the base of the saturated zone during supergene oxidation. Contours of the elevation of the paleopotentiometric surface based on this mineralogical transition show that the thickest portions of the unsaturated zone coincided with a silt-sand matrix diatreme breccia complex at Boyongan. Within the breccia complex, the thickness of the unsaturated zone approached 600 m, whereas outside the breccia complex (e.g., at Bayugo), the thickness averaged 50 m. Contours of the paleopotentiometric surface suggest that during weathering, groundwater flowed into the breccia complex from the north, south, and east, and exited along a high permeability zone to the west. The high relief (>550 m) on the elevation of the paleopotentiometric surface is consistent with an environment of high topographic relief, and the outflow zone to the west of the breccia complex probably reflects proximity to a steep scarp intersecting the western breccia complex margin. Stable isotope paleoaltimetry has enabled estimation of the elevation of the land surface, which further constrains the physiographic setting during supergene oxidation. Isotopic measurements of oxygen in supergene kaolinite from Boyongan suggest that local paleometeoric water involved in weathering had a δ 18 O composition of approximately –5.7 per mil. At the latitude of the southern Philippines, this value corresponds to Pleistocene rain water condensing at elevations between 750 and 1,050 m above contemporary sea level, providing a maximum estimate for the surface elevation during weathering of the porphyry systems. Physiographic reconstructions suggest that the deep oxidation profile at Boyongan formed in an environment of high topographic relief immediately east of a prominent (>550 m) escarpment. The high permeability contrast between the breccia complex and the surrounding wall rocks, coupled with the proximity of the breccia complex to the escarpment, led to a depressed groundwater table and a vertically extensive unsaturated zone in the immediate vicinity of Boyongan. This thick vadose zone and the low hypogene pyrite/copper sulfide ratios (0.6) at Boyongan promoted in situ oxidation of copper sulfides with only modest (<200 m) supergene remobilization of copper. In contrast, higher hypogene pyrite/chalcopyrite ratios (2.3) at Bayugo led to greater acid production during weathering and more complete leaching of copper above the base of oxidation. This process promoted significant (600 m) lateral dispersion of copper down the paleohydraulic gradient into the diatreme breccia comple, ultimately leading to the formation of an exotic copper deposit.

Abstract In the middle slope section of the Lower Congo Basin a late Miocene channel system was tracked on 3D seismic profiles over more than 350 km. Along its course, between shelf and basin, the system encountered four regional tectonic structures that induced local slope modifications, either by uplift or by subsidence. The turbidite deposits of this channel system were influenced strongly by these structures, in terms of both character and morphology. From the proximal to the distal part of this channel system, variations in parameters such as the sinuosity, the width and depth of basal incision, the presence of splay and levee deposits, the location of vertical aggradation zones and channel avulsion, all correlate with changes in longitudinal slope gradient. Thus, along a conventional sigmoidal slope, the convex regions are subjected to erosional processes whereas the concave regions are depositional. The direct relationship observed between sedimentary deposits and changing slope gradient highlights an important control in deep-water turbidite systems. This apparent response to local gradient changes on the slope suggests the existence of a sedimentary equilibrium profile similar to that defined for fluvial systems.

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Abstract The post-Eurekan pre-Pleistocene history of the Arctic Islands is poorly known because the stratigraphic record of this interval is limited mainly to the northwestern margin of the Queen Elizabeth Islands (Chapter 15). The lack or scarcity of preserved sediments in other areas indicates uplifts, which, in interaction with erosion and eustatic sea level changes, gave rise to the present landscape (cf. Thorsteinsson and Tozer, 1970). Middle or Late Tertiary positive movements were most pronounced within the Eurekan Orogen and a belt surrounding the Labrador Basin, and of lesser magnitude in the area south of the Eurekan Orogen. The uplift of the Eurekan Orogen represents isostatic adjustments to post-Eurekan erosion; the causes of the uplifts in the other areas are uncertain although a thermal model appears to be applicable to the mountains around Labrador Basin. The positive movements have been amplified to a minor extent by an overall drop in sea level since early Tertiary time (Vail and Hardenbol, 1979). The Holocene postglacial rebound will not be considered in this context because it merely tends to restore conditions that existed before the Pleistocene. Crustal thickening during the Eurekan Orogeny resulted in high topographic relief that produced the syntectonic clastic sediments of the Eureka Sound Group (Chapter 15). Post-Eurekan erosion must have caused an isostatic disequilibrium that was compensated by upward movement. The presence of high mountain ranges and matching negative Bouguer anomalies shows that the crustal roots of the orogen have not yet been eliminated by these processes. It is generally

Abstract The geomorphic development of the Canadian Cordillera is here considered as starting from Middle or Late Jurassic time when ancestral North America collided with the Intermontane Superterrane along its western margin. This, and a similar event later in the Mesozoic, produced two metamorphic and plutonic complexes, each of which are loci of high-grade metamorphism, rapid uplift, and vigorous erosion. These complexes, the Omineca Belt in the east and the Coast Belt to the west, constitute two of the five morphogeological belts in the Cordillera. Development of the Foreland Belt was an additional outcome of plate collision. With the growth of this belt in mid-Mesozoic to earliest Cenozoic time, a trellis drainage pattern developed, simulating that in the present Cordillera. Individual river courses of that time, however, bear little or no relationship to present drainage. The early erosion-products, together with material from the Omineca Belt to the west, contributed to an almost continuous apron of alluvial fans and associated deltaic deposits in the foredeep, east of the rising orogen. Widespread Early Cretaceous pediplanation, recorded by pre-Hauterivian to Albian erosion surfaces, coincided with a lull in magmatic activity throughout the Cordillera. Contemporaneous uplift and volcanism in the western Cordillera provided sources for rapid degradation and alluvial sedimentation in and adjacent to contemporaneous narrow seaways. Although transcurrent faulting and arc (Andean?) magmatism prevailed in part of the western Cordillera erosional and depositional activity west of the Foreland Belt subsided in Late Cretaceous and Paleocene time only to become vigorously rejuvenated in Early