AAPG Southwest Section

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Analyzing siliciclastic contant of ramp to rimmed carbonate slope deposits during relative sea level highstands


In the Delaware Basin, visible siliciclastic grains are present in various highstand slope and basin carbonate deposits, yet appear absent in others. We propose that a rimmed versus ramp shelf geometry may partly control the quantity and grain size of the detrital siliciclastic grains delivered to basin settings at times of relative highstand sea level. Samples were collected along detailed, measured sections through similar environmental facies of the Bell Canyon Formation (rimmed) and the Bone Springs Limestone/Cutoff Formation (ramp) to analyze the detrital silicate-mineral fraction weight percent and grain size.

Samples were treated with formic acid and hydrogen peroxide to remove carbonate and organics, respectively. Preliminary remaining-residue fraction from the rimmed system's Bell Canyon Formation averages 4.2% (proximally) and 6.5% (distally), while the ramp system's proximal Bone Springs Limestone averages 15.9% and the distal Cutoff Formation averages 7.1%, of the original rock weight. Note, the remaining-residue fraction demonstrates that the median grain sized in each distal interval of the two sections is relatively consistent, with each sample dominantly composed of grains 20 microns or smaller in diameter. Grain-size analysis of the proximal sections awaiting analysis.

Mineralogy of remaining-residue was determined by XRD and binocular microscope analysis. Some samples did contain significant sand-sized detrital quartz grains. More in the Bone Springs Limestone/Cutoff Formation than the Bell Canyon, and more in the proximal than distal sections. Higher remaining-residue fractions with more, and larger, quartz grains in the Bone Springs Limestone/Cutoff Formation appear to support our hypothesis.