Physical Properties by Geologic Unit in the Southern San Luis Basin, New Mexico

Physical properties of geologic units are important for geophysical interpretation because they provide the tie between lithology and geophysical fields. For gravity data, the applicable physical property is bulk density, which is the overall mass per unit volume of rocks, sediments, and their pore spaces. Bulk dry density is the mass per unit volume measured when the sample is dry. Saturated density is the mass per unit volume measured when the sample is saturated with fresh water, which has a density of 1,000 kg/m^3. The two values are commonly compared in order to determine the porosity of a sample, that is, the percentage of the volume that is composed of open space. Density is commonly reported in kilograms per cubic meter in the International System of Units (SI). For magnetic data, the applicable physical property is total magnetization, which is determined by the quantity of naturally occurring magnetic minerals in rocks and sediments, and the nature of their permanent magnetizations. Total magnetization is the vector sum of two components: remanent and induced (Blakely, 1995; Hansen and others, 2005). The induced component is a function of the quantity of magnetic minerals (commonly magnetite) and is a vector that is always oriented parallel to the present-day Earth’s magnetic field. The quantity of magnetic minerals is proportional to magnetic susceptibility, a property that can be measured from hand samples or outcrops. Measurements using these instruments provide values of volume magnetic susceptibility as opposed to mass magnetic susceptibility, which is commonly obtained in the laboratory. Values of magnetic susceptibility represent a ratio, and so are unitless, but the system of units must be specified for the values to be useful. Data reported here are in International System of Units (SI). The remanent component represents the vector sum of all permanent magnetizations held by the magnetic minerals, which have fixed directions irrespective of the ambient magnetic field (McElhinny, 1973). Remanent components that are generally aligned with or opposite to the present-day Earth’s field are considered to have normal or reversed polarity, respectively. The remanent component is determined from paleomagnetic laboratory measurements of oriented samples, none of which are reported here. However, the remanent components for volcanic units in the southern San Luis Basin are known to be significant, and commonly dominate the total magnetization (Grauch and Keller, 2004).

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