Eocene and Neogene volcanic rocks in the southeastern Nechako Basin, British Columbia: interpretation of the Canadian Hunter seismic reflection surveys using first-arrival tomography

Abstract

Geological investigation of the near-surface in the southeastern Nechako Basin is difficult. Shallow seismic reflection imaging is poor due in part to an extensive cover of Eocene and Neogene volcanic rocks. Outcrops of these volcanic rocks, and the primarily Cretaceous bedrock, are commonly obscured by Quaternary deposits and vegetation. Estimates of near-surface P-wave velocity are derived from the tomographic inversion of seismic first-arrivals, an effective tool when seismic imaging is poor. Tomographic model velocities are in agreement with sonic logs and laboratory samples, except for those from the Neogene Chilcotin Group. Cretaceous sedimentary rocks have velocities of ∼2800–4200 ms–1. The Eocene Endako and Ootsa Lake groups, which have velocities of ∼3000–4200 ms–1, are not distinguishable based on velocity. The velocity, the character (density, focus, and penetration depth) of rays, and ties with well and surface geology constrain the subsurface extent of the Endako Group adjacent to well b-82-C. The Chilcotin Group typically exhibits velocities (∼2400–3000 ms–1) lower than corresponding velocities from sonic logs (4500–5200 ms–1) and laboratory measurements (5000–5200 ms–1). These low model velocities may be due to the presence of high porosity, brecciated rocks near to the surface, in comparison with the other measurements that have focussed on lower porosity massive lavas. The lowest mean velocities, located to the southeast, are related to anomalously thick, high porosity, breccia-rich deposits of Chilcotin Group. This conclusion is consistent with the interpretation that the Chilcotin Group is thicker in paleo river valleys.

Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography

“…If originally deposited, the preservation of any Cretaceous sedimentary strata beneath these Tertiary rocks would depend on the level of erosion prior to Tertiary crustal extension. Hayward and Calvert (2009) found that Chilcotin Group volcanic rocks are characterized by unusually low velocities of 1.6-3.6 km/s in tomographic velocity models derived from the firstarrival tomography of short, <2.5 km, offset reflection lines shot by Canadian Hunter Ltd. Our results are consistent with their finding because we infer velocities of 1.4-3.6 km/s close to the surface where the surface lithology is identified as Chilcotin Group. However, laboratory velocity measurements on dry samples cored from massive flows of Chilcotin basalt yield velocities of 3.9-6.3 km/s with a mean of 4.8 km/s (Fig.…”

“…The significant overlap in P-wave velocity (3.6-5.3 km/s) between the Eocene Ootsa Lake and Cretaceous Taylor Creek groups indicates that they could be difficult to discriminate using P-wave velocity alone. The measured P-wave velocities of cores from Chilcotin volcanic rocks range from 3.94 to 6.28 km/s, and as noted previously by Hayward and Calvert (2009), these values are significantly greater than seismic velocities estimated at a larger scale in the field using seismic refraction tomography.…”

“…The survey was designed to mitigate, as far as possible, the negative effects of near-surface volcanic rocks, and to record long-offset first arrivals to obtain P-wave velocity models that can provide complementary lithological information where no reflections are observed. Using straight seismic profiles with a far offset of 2.55 km that were shot by Canadian Hunter Ltd. in the 1980's, Hayward and Calvert (2009) estimated near-surface seismic P-wave velocities to ϳ0.5 km depth beneath the Nechako-Chilcotin plateau using two-dimensional (2-D) tomographic inversion of first-arrival travel times. The 2008 Geoscience BC survey was designed, as discussed in more detail by Calvert et al (2011), to record first arrivals to source-receiver offsets as great as 14 km so that tomographic velocity models could be derived to depths of 2-3 km or more.…”

CrustalVSstructure in northwestern Canada: Imaging the Cordillera-craton transition with ambient noise tomography

“…The Nechako Basin, also an interior basin located in British Columbia, covers 75,000 km 2 . Because it is overlain by volcanic flows, less is known about the sedimentary structure, but sediment thickness is estimated to be ∼3 km in some places [e.g., Hayward and Calvert, 2009;Calvert and Hayward, 2009].…”

Seismic and gravity constraints on the crustal architecture of the Intermontane terranes, central Yukon

“…5a). Although such low velocities are not typically associated with volcanic rocks (values determined from core samples are commonly 4.0-6.5 km/s), first arrival tomography across the Nechako-Chilcotin plateau found velocities of 1.8-3.0 km/s associated with the Miocene-Pliocene Chilcotin volcanics (Hayward and Calvert 2009;Talinga and Calvert 2014). The explanation for such low velocities is unclear, but could be related to the presence of a variety of high porosity volcanic facies, which may in some cases occur above the water table.…”