Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon

Abstract

Convergent-margin basins (CMBs) are commonly associated with active arcs, and hence are rich in detrital zircon (DZ) whose ages closely reflect the timing of deposition. Consequently, maximum depositional ages (MDA) from DZ geochronology can be employed to resolve the stratigraphy and evolution of CMBs. Herein, we use DZ to revise the internal architecture of the lower Nanaimo Group, which partially comprises the fill of the (forearc) Georgia (or Nanaimo) Basin. Maximum depositional ages and multi-dimensional scaling of DZ age distributions are employed to determine chronologic equivalency of strata and assess sediment provenance variability within the pre-existing lithostratigraphic framework. The results are compared to a recently developed sequence stratigraphic framework for the lower Nanaimo Group. The basal lithostratigraphic unit of the Nanaimo Group, the Comox Formation (Fm), comprises strata that are neither time correlative nor genetically related. The three lithostratigraphic units directly overlying the Comox Fm (Haslam, Extension, and Protection formations) comprise strata with similar genetic affinities and MDAs that indicate deposition of these units was not always sequential and locally was contemporaneous. Through this work, we provide an example of how MDAs from DZ geochronology in CMBs can resolve basin-scale stratigraphic relations, and identify chronological changes in sediment provenance.

Along-strike variations in sediment provenance within the Nanaimo basin reveal mechanisms of forearc basin sediment influx events

“…2A). The lower Nanaimo Group is composed of both non-marine and marginalmarine strata (Mustard, 1994;Jones et al, 2018;Huang et al, C e n t r a l T r a n s e c t N o r t h e r n T r a n s e c t S o u t h e r n T r a n s e c t S a n J u a n T r a n s e c t et al, 2019) ( Fig. 2A).…”

“…The sediment provenance of the Nanaimo Group has been interpreted based on sandstone and conglomerate clast petrology (Pacht, 1984;Mustard, 1994;Katnick and Mustard, 2003;Brown, 2012), and U-Pb detrital zircon age distributions (Mustard et al, 1995;Housen and Beck, 1999;Mahoney et al, 1999;Brown, 2012;Englert et al, 2018;Huang et al, 2019). The adjacent Coast Mountains Batholith is considered the dominant sediment source for the basin, as evidenced by abundant plutonic detritus, volcanic lithic fragments in the Nanaimo Group sandstones, and Mesozoic detrital zircon ages (Mustard, 1994;Fig.…”

“…If the NCTS only contributed sediment to the southernmost part of the Nanaimo basin, an alternative source is needed for the small quantities of chert found within the lower Nanaimo Group exposed far from the NCTS (e.g., GabPRt, EX). It is possible that chert in the lower Nanaimo Group derived from minor chert units within strata of Wrangellia (Pacht, 1984;Brandon et al, 1986), which is thought to have formed basement highs that controlled sedimentation of the lower Nanaimo Group (Pacht, 1984;Mustard, 1994;Kent et al, 2019), as well as acted as a sediment source (Huang et al, 2019). The lack of chert in the upper Nanaimo Group in these regions could signal burial of Wrangellian basement during progressive onlap of the basal unconformity during deposition of lower Nanaimo Group (Johnstone et al, 2006;Kent et al, 2019 1-13) and potential source terranes in the NCTS (i.e., PDFs 14-24).…”

“…Only recently have large-n laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) detrital zircon techniques been used to unravel contentious ideas about the origin of Nanaimo basin strata Englert et al, 2018;Huang et al, 2019;Kent et al, 2019). However, a basin-wide investigation of spatial and temporal changes in provenance has the potential to answer numerous outstanding questions regarding sediment transfer along the Pacific margin of North America.…”

Late Middle Miocene Emergence of the Olympic Peninsula Shown by Sedimentary Provenance

“…Rocks of the Bonanza Arc yield U-Pb ages ranging from 168 to 202 Ma; associated volcaniclastic strata indicate that magmatic activity had started earlier in the Triassic, potentially as early as 230 Ma [73]. A similar 170-220 Ma zircon population is found in the age distribution of samples from Cretaceous forearc strata in the Georgia Basin, east of Vancouver Island [74]. This population is particularly prominent in samples from the Comox Formation of the lower Nanaimo Group in the southern part of the Georgia Basin, and is interpreted by Huang et al [74] as reflecting sediment sourced from the Bonanza Arc (Figure 7).…”