Application of Large-N Detailed Max Depositional Age (MDA) Calculation to Determine Sedimentary Rates of a Deepwater Conduit

Abstract

Constraining the high-resolution timing and duration of sedimentary processes within the geologic record is a considerable challenge in many basins. The calculation of maximum depositional age (MDA) from detrital populations of zircon age spectra is used to determine the timing of basin formation, and has aided in the correlation of larger stratigraphic packages devoid of high-resolution biostratigraphic data. Previously, MDA data sets have relied on relatively small samples of detrital zircons (n<100) to identify the youngest statistically significant sub-population of detrital zircons. Through the use of Laser Ablation Inductively Couple Plasma Mass Spectrometry (LA-ICP-MS) much larger sample sizes (n>500) can be efficiently analyzed, which allows for less abundant sub-populations of zircons to be resolved. In this study we use large-n detrital zircon datasets (n>300) to calculate MDAs, providing constraints on sediment deposition rates in a fore-arc basin deep-water channel system. The method is particularly successful in the area studied as units were deposited adjacent to an arc that was apparently continually active. The Campanian-Paleogene Nanaimo Group of B.C., Canada, was deposited in a forearc basin, west of the Coast Mountain Batholith. A record of long-lived deep-water sediment transfer is preserved on Denman and Hornby islands; sandstone- and conglomerate- dominated strata compose a composite sedimentary unit 20 km across and 1.5 km thick, in strike section. Eleven large-n (>300) U-Pb detrital zircons samples from the exhumed submarine channel system were analyzed to resolve the timing of sedimentary processes within the stratigraphic succession. From calculated MDAs, sedimentation rates are observed to increase upward through the stratigraphic succession from 60-100 m/myr to >500 m/myr. Initial phases of sediment bypass through the conduit can account for lower sediment accumulation rates. Subsequently, sedimentation rates increased significantly as the channel system developed levees and aggraded more rapidly. The 15-20 myr span of sediment transfer was punctuated, including a hiatus of ∼6 Ma. The large-n detrital zircon dataset provides unprecedented insight into long-term sediment routing, evidence for which is corroborated with field data.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016