PRELIMINARY RESULTS: COMPARING SILICICLASTIC CONTENT OF RAMP TO RIMMED CARBONATE SLOPE DEPOSITS DURING RELATIVE SEA LEVEL HIGHSTANDS

In the Delaware Basin of west Texas, visible volumes of siliciclastics are present in various highstand slope and basin carbonate deposits, yet appear absent in others. We propose that a ramp versus rimmed shelf geometry may be a control on the volume and grain-size of detrital siliciclastic grains delivered to basin settings at times of relative highstand sea level. Samples were collected along a detailed, measured section through similar environmental facies of the Bell Canyon Formation (rimmed) and Bone Springs Limestone (ramp) to analyze the detrital silicate-mineral fraction weight percent and grain size. Samples from both areas are dark, organic-rich micrite to wackestone, as confirmed by thin section analysis.

Samples were treated with formic acid and hydrogen peroxide to remove carbonate and organics, respectively. The remaining-residue fractions from the Bell Canyon Formation average 6.5% and range from 3-13% of the original rock weight. The Bone Springs Limestone’s remaining-residue fractions average 2.8% and range from 2-3% of the original rock weight. Furthermore, grain-size analysis of the remaining-residue demonstrates that the median grain size (D₅₀) in each interval of the two sections is relatively consistent and ranges from 6-12 microns (very fine to fine silt), with each sample dominantly composed of grains 20 microns or smaller in diameter. Most coarse silt and sand-sized grains from all samples are diagenetic products, such as silicified paleomicrobe skeletons, amorphous silica precipitate, iron oxides, or glauconite, as determined by binocular microscopic analysis. Some samples did contain significant sand-sized detrital quartz grains, most notably those of the Bone Springs Limestone. Note, the remaining-residue weight fraction includes not only detrital silicate-mineral fraction, but also these diagenetic products.

Preliminary results indicate that most of the indissolvable silicate-mineral fraction is dominated by silt. Sand-sized particles from the Bell Canyon Formation are almost exclusively diagenetic products. Sand-sized detritus is more common in the Bone Springs Limestone than the Bell Canyon Formation, implying that a rimmed shelf limits sand grains bypassing the carbonate platform during relative highstand sea level.