Completed projects in Mexico:

In late fall of 2008, we collected a suite of 35-40 mudstone samples from the uppermost part of the latest Cretaceous Upper Potrerillos Mudstone that crops out in the El Gordo minibasin of the La Popa basin in NE Mexico. These samples were analyzed using vitrinite reflectance and revealed an unexpected, and highly asymmetrical, thermal anomaly. This page shows the location map of La Popa basin (labeled "LP"), a geologic map, and cross-section. The El Gordo diapir is labeled "EG" on the map and is the southern diapir on the cross-section. Vitrinite sample localities and results are shown following the map images.

RESULTS

1. Google Earth images of the El Gordo salt withdrawal minibasin showing sample locations (upper) and vitrinite reflectance results (following figure). The diapir is located in the upper right and is encircled by a yellow line. Vitrinite values shown in yellow are presumed to be representative of regional values; orange and red values occur near the NW margin of the diapir and reach as high as 1.44 Ro; greens and blues are cooler and reach a low of 0.50 Ro near the SE margin of the diapir. We interpret high Ro values to represent elevated heat flow above an inclined salt diapir that plunges to the northwest, and the low values to the SE to be caused by an interpreted sub-diapir position. If our interpretations are correct, then they invalidate the assumed vertical orientation of the El Gordo diapir. If the diapir at El Papalote is derived from a similar position at depth, then overturned halokinetic sequences that crop out along the eastern edge of the diaper, which have typically been ascribed to a ballooning of the diaper, may actually be caused by the diapir over riding the strata. Another important finding is that the scale of the anomaly is larger than what numerical models predict.

2. Sandstone samples of the K-T tsunami deposit known as the Delgado sandstone that were collected from the same localities as our vitrinite samples in the El Gordo minibasin (see previous figure) were analyzed using QEMSCAN, an automated SEM with backscatter chemical analysis and via standard petrography. Given the wide variability in vitrinite values that we documented in the adjacent Lower Potrerillos Mudstone, we hypothesized that:

   1. these reservoir analogue sandstones experienced different diagenetic histories, and that samples from areas that had depressed vitrinite results did not undergo quartz cementation whereas those from areas that had high vitrinite values would have significant amounts of quartz overgrowths. We impregnated samples with blue epoxy and analyzed them using QEMSCAN and standard petrography. What we found were that samples contained primarily calcite cement or original fine-grained matrix material between the framework grains. We found essentially no quartz cementation in any of the Delgado Sandstone samples based on the QEMSCAN results as well as via standard petrographic analysis.

   2. that feldspar compositions would vary depending on location; that those from regions that had lower vitrinite values would contain a greater variety of feldspar compositions and that those from areas with higher vitrinite values would have feldspars with more albitic composition. Our results showed no significant variation in feldspar composition across the entire basin.

   3. that ankerite cements might be present in the area that had high vitrinite values. The QEMSCAN data revealed no ankerite cements anywhere in our samples.

3. We began using remote sensing methods to model modern heat flow around exposed diapirs. Our initial efforts were around the Ribera Alta diapir in northern Spain as well as on the El Gordo diapir in La Popa basin, NE Mexico. This project used remotely sensed thermal data collected in areas adjacent to the diapir and compared them to areas with similar geology but where no salt structures are present. The data were modeled in order to remove diurnal, seasonal, glacial-interglacial heat fluxes in order to assess any modern-day thermal variability around the salt structure. This work was part of Nick Downs M.S. thesis which is located elsewhere on the website. In short, remote sensing efforts on the Ribera Alta diapir were inconclusive because of the abundant vegetation in the area. However, significant thermal signatures were modeled at La Popa; see Nick’s thesis for full details. Dr. Scott Nowicki supervised this portion of the project.

ONGOING/FUTURE WORK

4. We are in the beginning stages of separating clay mineral species from our Potrerillos Mudstone samples for subsequent XFD analysis. Previous research has shown that original smectitic clays undergo dehydration reactions as temperatures increase and that they convert to illite. Our hypothesis is that smectite/illite ratios may change depending on the proximity to the diaper. We will prepare and run the samples in UNLV’s XXL: XRF/XRD lab.