Geophysical-geological analysis of Fairfax County, Virginia

Aeromagnetic, aeroradioactivity, and gravity surveys of Fairfax County, Virginia, are interpreted with the aid of existing geologic data and local geophysical measurements. Highly magnetic metasedimentary rocks of the Peters Creek Schist and Sykesville Formation produce most of the magnetic anomalies of Fairfax County. In contrast, the granitoid rocks of the Occoquan pluton are relatively non-magnetic and are characterized by broad magnetic and gravity lows. Linear magnetic highs without corresponding gravity anomalies, plus samples from a test well, indicate that Peters Creek Schist underlies the east side of the Triassic-Jurassic Culpeper basin. The continuation of the Piney Branch mafic-ultramafic complex beneath the basin is indicated by a gravity high. Basement lithology beneath the Coastal Plain sediments is interpreted by the following anomaly combinations: Sykesville Formation - magnetic high with no gravity expression; Chopawamsic Formation -magnetic and corresponding gravity high; Occoquan granitoid rocks -magnetic and corresponding gravity low.

Analysis of a detailed gravity profile across a diabase body near Herndon indicates that the intrusive is a sill which is partly discordant with the sedimentary rocks of the Culpeper basin. The limbs of the horseshoe-shaped exposure dip gently toward the center; the west limb, 500 m thick, dips 15° east and the east limb, 250 m thick, dips west 3° or less. A second gravity profile across a similar diabase body near Centreville suggests that part of its western limb consists of a narrow dike-like body projecting upwards from a 200 m-thick buried sill. Both of these bodies apparently have a spoon-shaped structure with a discordant northern edge which was probably controlled by faulting. Similarities between these intrusives apply also to the small Boyds diabase in Maryland. The three diabase bodies are aligned, have similar magnetic expressions, and abut the edge of the Culpeper basin, suggesting that they were intruded under similar structural settings at about the same time. The northward decrease in size of these intrusives may be a consequence of the inward-dipping shape and a greater degree of uplift and deeper erosion to the north. By this reasoning, the Boyds body represents only the basal remnant of a largely eroded structure with the Centreville body preserving more of the original form.

A ground magnetometer survey and an aeroradioactivity survey indicate the presence of additional zones in the thermal metamorphic aureole in the sedimentary rocks around the large diabase bodies. A 150-300-m-wide inner magnetite-rich zone above and below the Herndon sill is inferred from anomalies on a detailed ground magnetic profile. Aeroradioactivity data show a rim of high values outside the geologically mapped aureole of both the Herndon and Centreville bodies. This suggests a subtle outer zone slightly enriched in thermally expelled radio-elements.

Deep resistivity soundings in the Culpeper basin indicate that the sedimentary rocks may reach a thickness of about 3600 feet (1100 m) at the western edge of the county.