Site M0077: Open Hole

Borehole Seismic Observations From the Chicxulub Impact Drilling: Implications for Seismic Reflectivity and Impact Damage

“…The sonic compressional log velocities through upper sections of post-impact Cenozoic sediments above 585 mbsf are consistent with depth trends seen elsewhere in shallow and primarily carbonate columns (e.g., Eberli et al, 2003;Japsen, 1998). Depths to 590 mbsf had initially been informally separated into two distinct zones from 47.5 m to 280 mbsf and 280 to 590 mbsf on the basis of average VSP transit time P-wave speeds of 2.285 km/s ±14 m/s and 2.567 km/s ±4 m/s, respectively (Gulick, et al, 2017d). The reason for the subtle Figure 3.…”

“…The sonic compressional log velocities through upper sections of post‐impact Cenozoic sediments above 585 mbsf are consistent with depth trends seen elsewhere in shallow and primarily carbonate columns (e.g., Eberli et al., 2003; Japsen, 1998). Depths to 590 mbsf had initially been informally separated into two distinct zones from 47.5 m to 280 mbsf and 280 to 590 mbsf on the basis of average VSP transit time P ‐wave speeds of 2.285 km/s ±14 m/s and 2.567 km/s ±4 m/s, respectively (Gulick, et al., 2017d). The reason for the subtle difference between the two zones is unknown but may result from mechanical changes resulting from the transition of biogenic opal‐A to opal‐CT (e.g., Guerin & Goldberg, 1996; Ishii et al., 2011; Meadows & Davies, 2009), the latter indicated by X‐ray identification of significant modal proportions of α ‐quartz, cristobalite and tridymite in the core Section 505‐585 mbsf (Gulick et al., 2017e, Table T8).…”

“…The reason for the subtle difference between the two zones is unknown but may result from mechanical changes resulting from the transition of biogenic opal‐A to opal‐CT (e.g., Guerin & Goldberg, 1996; Ishii et al., 2011; Meadows & Davies, 2009), the latter indicated by X‐ray identification of significant modal proportions of α ‐quartz, cristobalite and tridymite in the core Section 505‐585 mbsf (Gulick et al., 2017e, Table T8). This variation may alternatively be due to other changes in the rock composition as seen in the abrupt change in the natural gamma radiation log (GR) that may indicate greater concentrations of clay minerals above 275 mbsl (Gulick et al., 2017d, Figure F1). Anomalously low V P (both log and VSP ∼1.800 km/s) exist at ∼230–280 mbsf and this zone was likely subject to karstification as suggested by the appearance of large ∼10 cm diameter vugs in the ultrasonic televiewer images.…”