Department of the Interior -- U.S. Geological Survey
Deep Drilling - - Spring and Summer 2001
The Chesapeake Bay Impact Crater Project
Introduction
The U.S. Geological Survey and allied federal, state, and local agencies drilled two important deep test holes into the buried Chesapeake Bay impact structure between March and September 2001. Core drilling was completed in May at a site in western Mathews County, Virginia, and a second corehole was completed in early September in southern Mathews County. Information generated by these investigations is providing an improved understanding of the impact crater in the Middle Peninsula of Virginia as well as the crater's effects on regional ground-water flow and quality.
USGS field investigations of the Chesapeake Bay impact structure are conducted in cooperation with the Hampton Roads Planning District Commission, the Virginia Department of Environmental Quality, the NASA Langley Research Center, the Geology Department of the College of William and Mary, the Mathews County Board of Supervisors, and the Mathews County Land Conservancy. Funding for core drilling in Mathews County was provided by the Hampton Roads Planning District Commission, the Virginia Department of Environmental Quality, and the USGS.
North Corehole, Mathews County, Virginia
A deep corehole was drilled along highway 14 between the villages of North and Foster in western Mathews County between March 17 and May 12, 2001. This corehole penetrated the crater-fill deposits and overlying post-impact deposits to a total depth of 1,427.5 feet. The test hole was continuously cored, and sample sets were collected for lithologic, mineralogic, paleontologic, and hydrologic analyses. The following updates issued during the course of the drilling at North provide additional information:
North Corehole Update -- April 17, 2001
Drilling at the North drill site reached a depth of 1,127 feet on April 16, 2001. The top of the impact-crater materials is near a depth of 740 feet. Post-impact sediments of late Eocene, Oligocene, Miocene, and Pleistocene(?) ages are present above the crater deposits. Thus far, the crater-fill section consists primarily of small to large blocks of pre-impact sedimentary units "floating" in a muddy-sandy matrix. Blocks of Cretaceous(?) sediment in excess of 30 feet in apparent diameter have been intersected. An interim set of geophysical logs was run in the corehole on April 17.
North Corehole Update -- May 3, 2001
At the end of work on May 2nd, the drill bit reached a depth of 1,210 ft at the North site. Drilling had been suspended in late April to allow the necessary change from the larger diameter HQ rods and bit to a smaller diameter NQ coring system.
North Corehole Update -- May 15, 2001
Core drilling at the North site ended on May 8, 2001, and all USGS operations at that site were completed on May 12, 2001. The final depth of the corehole was 1,427.5 ft. The drilling rods became trapped in the corehole on May 9th and could not be recovered, except for a short section of rods at the top of the hole. A natural gamma log was collected through the drilling rods on May 10th, and a suite of open-hole geophysical logs was collected in the uppermost part of the corehole on May 11th. The geologic section in the lower part of the core consists of minimally disrupted Cretaceous sediments.
Bayside Coreholes, Mathews County, Virginia
Core drilling to a depth of 2,390 ft was conducted at the Bayside site, southern Mathews County, between late June and early September, 2001. The Bayside site is located at the southern end of route 14, about one-half mile south of the village of Bavon. The site is owned by the Mathews County Land Conservancy. Core drilling began on June 30 and continued, with one lengthy break, to a depth of 1,160 ft on July 19. At that time, the bit and rods became "sand-locked" in the corehole. After substantial efforts failed to remove the rods, this corehole (Bayside No. 1) was abandoned. The drilling of a second corehole at Bayside began immediately with the installation of surface casing. After the casing was installed, work was suspended until August 13. Drilling resumed on that date and continued, without sampling, to the previous depth of 1,160 ft. Coring in Bayside No. 2 started at that depth on August 18 and continued without significant delays until September 3 when the final depth of 2,390 ft was reached. Impact-generated and impact-modified sediments of the Chesapeake Bay impact structure were encountered in the Bayside cores between depths of about 915 ft and 2,326 ft. Below 2,326 ft, the section consists of medium-grained granite. No geophysical logs were run in Bayside No. 1. A suite of geophysical logs was run in Bayside No. 2 although not all of the probes reached the bottom of the hole. The following updates issued during the course of the drilling at Bayside provide additional information:
Bayside Corehole Update -- June 8, 2001
The second Mathews County corehole will be drilled at the Bayside site near the southern terminus of highway 14 in the Bavon area. Site preparation will begin on June 18, 2001, and coring operations will begin about one week later. This corehole is planned to penetrate the entire thickness of the post-impact and crater deposits and intersect crystalline rock at a depth below 2,100 feet. This drill site is located closer to the center of the crater than previous project coreholes and likely will reveal substantial new information about the physical character and formative processes of the crater.
Bayside Corehole Update -- July 16, 2001
Site preparations began at Bayside on June 18-21 with the installation of a perimeter silt fence and a water-supply well. The drill rig arrived at the site on June 25th. Following equipment setup and maintenance on June 26-27, drilling began about mid-day on June 28th. The drillhole was "washed" down to a depth of 163.5 ft by the late afternoon on June 29th, and six-inch-diameter steel casing was cemented in place to that depth on the evening of the 29th. Cuttings samples were collected from the 0- to 163.5-ft interval but no core. Coring began at the bottom of the casing on June 30th, and a depth of 480 ft was reached by the end of work on July 2nd. Operations at the Bayside site were suspended from July 3rd through July 12th for the Fourth of July holiday and because of logistical limitations. The drill crews and geologists returned to the site on July 13th, and around-the-clock ("24/7") coring operations began on July14th. Coring had proceeded to a depth of 680 ft within post-impact Oligocene marine sediments as of mid-morning on July 16th.
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