Lake Wanapitei, located within the Southern Province of Ontario, Canada, provides the setting for a unique study of an impact crater situated within a shield environment. Evidence for the 7.5-km-diameter Wanapitei impact includes a circular Bouguer gravity low centered over the central area of the lake and features of shock metamorphism in samples of glacial drift found on the southern shores. Geophysical studies of craters in hard-rock environments are often limited by the lack of markers used for exploration; this may be overcome with the use of the large igneous dike swarms that characterize Archean terrains. The 1.2 Ga Sudbury dike swarm predates the impact that is suggested to have generated Lake Wanapitei and provides the setting for a study to constrain the size and location of the impact crater. The swarm is clearly visible on aeromagnetic maps as high amplitude, linear features, suggesting they could be used as vertical markers indicative of structural changes having an effect on target rock susceptibilities. To fully establish the size of the crater, a total field magnetic map was produced to trace the Sudbury dikes through the proposed crater center. A gap in their signature, expressed as a 100 nT low, 2–3 km in width, constrains the size of the crater to <5 km. Numerical modeling suggests that a crater of this size will demagnetize target rocks, producing a low in the total magnetic field, up to a maximum diameter of 3 km. Dikes

Abstract The continental lithosphere of southwestern Gondwana, comprising the southern part of South America and southern Africa, was largely assembled before the end of the Proterozoic. Geologic studies indicate that the basement anisotropy that controlled the development of Phanerozoic basins was established by Neoproterozoic–Early Cambrian tectonism. This tectonism reactivated the older terrane boundaries or cut across them. The backbone linking the system of Pan-African and Brasiliano basins was a system of northeast-trending structures. There were four areas of pronounced Neoproterozoic–Early Cambrian basin subsidence in the study area: the Chiquitanas trough in Bolivia, the Puncoviscana basin in Argentina, the Dom Feliciano-Ribeira basins in Brazil, and the Damara-Nama basin complex of southern Africa.

Abstract Although glaciated basins are usually associated with nonproductive, poorly sorted strata, hydrocarbons occur in several late Paleozoic glaciated basins of central and southern South America. In Bolivia, the Chaco-Tarija basin has commercial production from more than 30 fields in glacially influenced submarine channel systems (Palmar, Santa Cruz, and Bermejo fields) that accounts for about 60% of current national reserves. Correlative deposits in Argentina host the Campo Duran and Madrejones oil fields. In Brazil, the Paraná basin has significant but as yet subcommercial gas shows in thick marine turbidite sandstones of the glacially influenced Itararé Group. The Chaco-Paraná basin of Argentina is one of the largest onshore targets for exploration in South America, but it is virtually untested. Glacially influenced foreland basins of Argentina (Tepuel and Paganzo-Maliman) contain complex glacigenic stratigraphies of interbedded tillites and poorly prospective sandstones. In contrast, the glacially influenced marine infills of intracratonic basins in Brazil (Paraná), Bolivia, and Argentina (Chaco-Tarija and Chaco-Paraná) contain thick sequences of pebbly mudstones and regionally extensive reservoir quality sandstones. The key to the occurrence of good reservoirs and associated trapping mechanisms in these intracratonic basins is the interplay of sediment supply, regional tectonics, and relative sea level changes. Glacial scouring of extensive cratons by ice sheets resulted in the delivery of huge volumes of glaciofluvial sand to deltas. Structural control of drainage patterns on the craton by basement lineaments resulted in persistent sediment sources and depocenters. Frequent earthquake activity along reactivated basement lineaments resulted in downslope mass flow of deltaic sediments and the deposition of thick, amalgamated sand turbidites (reservoirs). Pebbly mudstone seals most likely record higher relative sea levels, resulting from basin subsidence, and deposition from suspended sediment plumes and icebergs. Source rocks are provided by Devonian and Permian shales. This model may be applicable to other parts of Gondwana that contain thick, prospective sandstones in glacially influenced intracratonic basins.

ABSTRACT Foraminifera are common in the glaciomarine Yakataga Formation of the eastern Gulf of Alaska and can provide key insights into the depositional history of Late Cenozoic glaciomarine paleoenvironments in the northeast Pacific Ocean. Lithologic evidence of tidewater glaciation consists of two main intervals of diamictites and sediments containing ice-rafted debris. The first is in the basal Yakataga Formation and is of latest Miocene age, while the second consists of 2-4 km of late Pliocene-early Pleistocene glaciomarine sediment in the upper Yakataga Formation. A distinctive feature of this latter interval is megachannels up to 400 meters deep and several kilometers wide. Megachannels cut into, and are filled with, a variety of lithofacies, including massive and stratified diamictites, thinly interbedded turbidite sandstones and mudstones, massive to laminated mudstones and crudely stratified conglomerates. These megachannels have been identified as possible paleofjords by some investigators but may also represent glacially influenced sea valleys, similar to the modern Yakutat Sea Valley or Bering Trough. A study of foraminiferal biofacies and sediments provide a paleoenvironmental framework for evaluation of the origin of the megachannels. Channel-fill successions begin with conglomerates overlain by fine-grained turbidites and mudstones. The turbidites pass upwards into massive and stratified diamictites deposited predominantly by sediment gravity flow processes. The turbidites and mudstones contain faunas characterized by Epistominella pacifica, agglutinated taxa and contain planktic foraminifera. These faunas represent upper bathyal water depths (150-500 m). Some diamictites contain sparse faunas dominated by Elphidium excavatum clavatum and represent neritic water depths. Near the margins of megachannels, vigorous downslope gravity processes are reflected by gravel beds and upper bathyal biofacies containing greater numbers of displaced shallow water taxa (particularly Elphidium excavatum clavatum ), including rare innermost shelf taxa (e.g., Elphidiella oregonense). The distribution of foraminiferal biofacies suggests water depths for channel-fills consistent with the amount of channel incision (100’s of m) and does not suggest restricted or silled conditions as seen in many modern fjords. Thus, the megachannels most closely resemble the glacially-influenced sea valleys found on the modern Gulf of Alaska continental margin.

Coastal mountains in the northeastern Gulf of Alaska expose continuous, along-strike sections over many tens of kilometers through the 5-km-thick infill (Yakataga Formation) of a glacially influenced active margin basin. The basin has been thrust and uplifted as a result of continuing compression between the underlying Pacific and North American plates. The Yakataga Formation is the best exposed and most complete late Cenozoic record of cool temperate and glacially influenced marine sedimentation in the world. Glacial marine sedimentation began during the late Miocene and is recorded in lowermost Yakataga strata exposed at Yakataga Reef by the abrupt arrival in a deep basin of turbidites and chaotically bedded debris flows. Debris flows, as much as 19 m thick are composed largely of glacial debris brought down to sea level by tidewater glaciers. A depositional setting characterized by a narrow shelf terminating in a steep slope and deep water, and subject to frequent downslope mass flow events is indicated. Overlying Yakataga strata exposed at Icy Bay are characterized by interbedded turbidites, diamictites, and shallow marine sandstones; these facies probably record progradation of a continental slope by mass flow processes in response to high rates of sediment supply from glaciers draining rapidly uplifting (1 to 10 m/yr) and eroding coastal mountains, and earthquake activity. Seismic and outcrop data show that the slope experienced multiple episodes of syndepositional compressional folding, resulting in a pronounced structural influence on sedimentation style. Uppermost Yakataga strata exposed on Middleton Island are dominated by thick “rain-out” diamictites resulting from iceberg transport of coarser debris and the deposition of muds from suspended sediment plumes in an outer shelf setting. Graded gravel facies record the infilling of submarine channels similar to the valleys that traverse the modern Gulf of Alaska shelf; coquinas indicate episodic sediment starvation. Boulder pavements record repeated surge-like ice advances to the outer continental shelf. By this time (late Pliocene to Pleistocene) a low-relief subsiding shelf was established in the Gulf of Alaska on which a high-resolution record of sea-level change, tectonism, and glaciation was preserved; deposition rates may have been as high as 10 m/ky. The single most important influence on sedimentation in the late Miocene to Pleistocene Gulf of Alaska, especially in allowing the preservation of a thick active margin basin fill in a compressional tectonic setting, has been, and continues to be, the abundant production of meltwater from temperate tidewater glaciers and associated sediment from rapidly uplifting (10 m/ky) coastal mountains.