A NEW TECHNIQUE IN PALEOMAGNETIC SAMPLE ACQUISITION

The standard method of obtaining oriented cores for paleomagnetic analysis is to use a Pomeroy or similar drill. The drill cores the rock, leaving the sample in place for orientation. The sample is then popped out. While this method works very well for hard and well-cemented rocks, it does not work for poorly-cemented sedimentary rocks or paleosols. If samples will be analyzed by AF demagnetization, a plastic box can be pushed into the soft sediment and oriented. However, if samples will be analyzed by thermal demagnetization, the box method will not work.

Morrison Formation rocks consist primarily of paleosols and poorly cemented sandstones and siltstones. The standard method cannot be used to recover samples here. Some of the harder claystones can be drilled without water. Those that cannot are either too soft to drill, liquefy, or break off in the drill as it is coring. In addition, paleosols in my section are characterized by high smectite content. The water needed to lubricate the drill causes massive swelling of this clay and ultimate loss of the sample.

I developed a new technique that has produced viable samples for paleomagnetic analysis. PVC piping is cut into 6” sections and one end is sharpened. The sharpened end of the pipe is laid against the rock and a piece of wood is laid against the blunt end to prevent damage to the pipe. A sledge or dead-weight mallet is used to hammer the pipe in. The PVC pipe was strong enough to be driven into the rock without shattering or splitting almost all of the time. The orienter is then placed in the end of the pipe, and the azimuth and hade read off of it. The orienter line is drawn directly on the surface of the pipe, and the sample—still in the pipe—is removed from the surrounding rock.

The sample is removed from the PVC pipe in the lab. Because of the fragmentary nature of the rock, the sample must be coated in Fiberfrax to survive the thermal demagnetization process. This is done in a series of steps to allow the orientation mark on the pipe to be transferred to the outside of the Fiberfrax. Preliminary results show that samples collected in this way have a pattern of reversals indicating original paleomagnetic signature. This technique is beneficial to paleomagnetic sampling because it allows thermal demagnetization to be conducted on rocks that previously could only be analyzed using AF demagnetization.