The Tethyan Pacific during Late Jurassic and Cretaceous times
Abstract During Mesozoic times, the Pacific Ocean occupied almost an entire hemisphere, plus a great wedge of ocean that lay between eastern Gondwana and Laurasia and extended westward from the Pacific into the gradually widening Tethys sea in the Mediterranea, Atlantic, and Gulf of Mexico. As the Atlantic widened, the Pacific narrowed apace, and the Pacific Plate expanded at the expense of bordering plates. During the Late Jurassic and Cretaceous, connections between the Atlantic and Pacific, via passageways between small continental plates in Central America, gradually widened, permitting a globe-circling, mainly west-flowing current system in low latitudes. Except for tectonic slices preserved along the adjacent American and Asian continents, all pre-Jurassic Pacific oceanic sediments have been subducted. Middle and Late Jurassic oceanic crust is inferred from magnetic data, and one drill hole reached Middle Jurassic crust. Pacific Cretaceous pelagic sedimentary rocks are typically nannofossil limestone above the calcite compensation depth and brown, zeolitic clay and radiolarian mudstone below. Chert is common as nodules in both limey and clayey sediments, and siliceous sediments appear to have been deposited over a wide band of latitudes in the Cretaceous. Lower Cretaceous black shale is present on several plateaus, at paleodepths of 500–1500 m, but contemporaneous sediments in adjacent basins are oxidized. Cretaceous vulcanism was on a stupendous scale: in the western equatorial Pacific, a region of about 4x106 km2, centered on the Ontong Java Plateau, is covered almost entirely by mid-plate tholeiitic flood basaltic flows and sills, probably emplaced mainly during the earliest Aptian. Emplacement of these huge masses in a relatively short time may have had important consequences not only for global sea level, but also for the chemistry of the oceans and atmosphere. A striking link to Tethyan facies in the other hemisphere is the presence of shallow-water platform and reefal sediments on the summits of many Pacific Cretaceous seamounts. Several dozen Early Cretaceous seamounts built to heights above sea level, and doubtless many more existed on crust now subducted along the margins of the Pacific. As the seamounts subsided, shallow-water rudist/coral platforms gradually covered them, eventually forming atolls. The platforms are the Pacific expression of the “Urgonian” platforms of the Tethys. Many—perhaps most—of the Pacific Early Cretaceous platforms drowned in the latest Albian, and are guyots today. New volcanic chains rose later in the Cretaceous, but there is a gap in rudist reef development from the Cenomanian through the Santonian. Campanian and Maestrichtian rudist reefs flourished in equatorial latitudes, but hermatypic reefs appear to have been absent during the Paleocene.