CENOZOIC ACCUMULATION HISTORY OF A PACIFIC FERROMANGANESE CRUST

To investigate the growth rates and absolute time stratigraphy of mari ne hydrogenetic ferromanganese encrustations, we performed Be-10 profi ling and 'Co chronometry' of crustal layers, as well as Sr-87/Sr-86 an d deltaO-18 analysis of phosphatised limestone (francolite) within a a pproximately 9.5 cm thick ferromanganese crust from Schumann Seamount in the Hawaiian Archipelago. Together with microfossil stratigraphy, o ur results indicate that some seamount crusts greatly exceed the commo nly accepted Miocene maximum age, in this case probably approaching th e Cretaceous age of the seamount. In addition to the unconformity at t he crust-substrate boundary, at least eight major disconformities are indicated in the Schumann Seamount crust which probably represent depo sitional hiatuses or episodes of crust erosion. Three of the six upper disconformities can be placed at the Plio-Pleistocene, Middle Miocene and Paleocene-Eocene based on Be-10, microfossil and Co chronometer e vidence. Sr-87/Sr-86 and deltaO-18 values of purified francolite from an inclusion-rich layer between the depths of 44 and 49 mm suggest app arent ages that approach those of Eocene-Late Paleocene microfossils r eported in overlying layers, whereas francolite vein infillings in the lower part of the crust and in the basaltic substrate yield values th at, if interpreted as ages of phosphatization, suggest a minimum Oligo cene age. Paleotracking suggests the phosphogenesis observed here and on other Central Pacific seamounts could not have resulted from upwell ing enhanced productivity associated with equatorial divergence if the Oligocene and Middle Miocene isotopic ages reported here and elsewher e are correct; however, a maximum Late Paleocene age for the phosphoge nesis, consistent with the stratigraphy, would place these seamounts w ithin 10-degrees-N of the equator. Paleotracking also suggests northea st tradewind transport of aluminosilicates in the Cenozoic, in agreeme nt with other evidence for the antiquity of this ferromanganese crust.