Post-Impact Thermal History and Thermal Maturity in the Chesapeake Bay Impact Structure, Virginia, Based from Downhole Vitrinite Reflectance
Vitrinite reflectance data from the 1766-m ICDP-USGS Eyreville corehole (2005-06) in the central crater moat of the late Eocene Chesapeake Bay impact (CBI) structure, coupled with heat transport modeling, indicate effects of advective and conductive heating. Maturity of post-impact coastal plain (CP) sediments (0-444 m) is typical for the region (0.2-0.32%Ro). However, maturity in underlying syn-impact marine resurge breccias and avalanched sediment megablocks is above background. From 444 to 525 m depth, reflectance increases, then remains constant (~0.44%), indicative of vertical advective fluid flow, down to 1096 m, the top of an allochthonous granite block. Below the granite, reflectance increases, 0.47% to 0.59%Ro, in a thin sand and crystalline block interval (1371-1397 m), due to conductive heat from underlying suevites and clast-rich impact melt rocks (1397-1474 m). Reflectances in the uppermost suevite are 1.2-1.4%. Modeling the Eyreville suevite as a 390oC cooling “sill” accompanied by compaction-driven vertical fluid flow (0.046 m/yr) from the suevite and deeper 120oC basement brines upward through sediment blocks/breccias for 10,000 years closely reproduces measured reflectance values. The results suggest that compaction-driven fluid flow can be an important process in distributing heat in impact structures, particularly marine impact structures having undercompacted sedimentary fill overlying fractured basement rocks.
Data from the Cape Charles test hole (central uplift) show a similar isoreflectance section in the upper sediment breccia with reflectance exponentially increasing just above crystalline-clast breccias with suevite. Previous corehole data from the outer annular trough revealed no impact-related thermal effects.
Oil test drilling in the Atlantic CP, 1944-71, found no petroleum/gas systems; the CBI event did not improve local petroleum potential. Although the impact enhanced basement fracture permeability, syn-impact resurge breccias are clay-rich with limited permeability. Pre- and post-impact Upper Cretaceous/Tertiary marine formations are organically lean; kerogen in Lower Cretaceous fluvial formations in the impact region is Type IV. Cored syn-impact sedimentary crater fill is still immature. Lean, overmature (>4%Ro) black shale clasts in suevitic breccias are assumed pre-Mesozoic since they are unlike CP sediments or organic-rich, gas-prone (<2.5%Ro) onshore Triassic Richmond and Taylorsville basins at the same latitude.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009