Table 1.

Summary of dating of sills and flows in the Wichianburi sub-basin

Tectonic phaseIntrusion typeIntrusion age (Ma)Group
Phase 3 (3–0 Ma)Amygdaloidal basalt flows (2 Ma)<3 (S)GJ
Shallow sills
Phase 2 (16–3 Ma)Gabbro–anorthosite–diorite (AF, VBF, Au)11.6 (K–Ar dating) (HG)GI
Sub-ophitic gabbro (PN)12.7–12.81 (K–Ar dating)GH
Diabase and diorite (LG)15.6–16.1 (K–Ar dating)GG
Sub-ophitic gabbro (AF, PB)<16GF
Cross-cutting sillGD
Amygdaloidal basalt flow and related sills16 (S)GE
Phase 1 (26?–16 Ma)Amygdaloidal basalt flow (An76–An48)18 (S)GC
Undated sills imaged on seismic sections<23GB
Glassy basalt (An62–45)24 ± 1.0 (40Ar/39Ar dating)GA

Phase 1, east–west extension; phase 2, NW–SE extension; phase 3, post-rift extension (see Fig. 3). Dating and igneous type from Barr & Cooper (2013) and Remus et al. (1993).

AF, presence of alkali feldspar; Au, high Al2O3 (11–12%) augite compared with 2–6% Al2O3 augite in other intrusions; HG, high gamma ray log signature; LG, low gamma ray log signature; PB, broad range of plagioclase compositions (An50–An68–05); PN, narrow range of plagioclase compositions (An62–An37); S, age based on dating of surrounding sediments; VBF, very broad range of feldspars An70 to anorthoclase.