Question

44 Historical Geology CARLSBAD CAVERNS, NEW MEXICO Exercise 1-4 This exercise gives you the opportunity to interpret a carbonate sedimentary environment. You are working in the business office of Sage Brush Electric Company in Alpine. Texas. To supplement your income, you are an active land speculator in southeastern New Mexi- co and in the adjacent west Texas region. During a lunch break, you overhear a lineman for the company relating the possible discovery of a large system of caverns that may be even larger than those in Carlsbad Caverns National Park. He also mentions an approximate lo- cation. As soon as possible, you get the land ownership plots for the area immediately sur- rounding the lineman's discovery. In college you took geology, and you know that the west Texas region is famous for a large structure called the Permian Reef. In your old notes you find a cross-sectional sketch of the Permian Reef. Immediately you begin to compile per- tinent facts on the Permian strata from the numerous dry oil tests in the region. The three objectives of the task before you are to (1) identify the distribution of the often-cavernous Capitan Limestone in the subsurface, (2) define the orientation of the main reef limestone mass, and (3) locate any areas of high porosity within the reef mass that could form caverns. Once these objectives have been achieved, you can begin your specu- lative land purchases with some assurance of potential success. DATA 1. A cross section of the Permian Reef (see Fig. 1.27). 2. Localities where data were collected (see Fig. 1.28). 3. Lithologic descriptions of the Permian strata at each locality. SAMPLES 1. Anhydrite, white, sparsely fossiliferous, contains some salt. 2. Shale, grey, few fossils, some anhydrite. 3. Anhydrite, white, sparsely fossiliferous, some dolostone. 4. Limestone, grey, very fossiliferous, very porous. 5. Limestone, grey, very fossiliferous, very porous. 6. Shale, grey, a few fossils, 7. Limestone, grey, very fossiliferous, some dolostone and anhydrite lenses. 8. Limestone, grey, fossiliferous. 9. Limestone and shale, black to grey-black, clastic, very fossiliferous. 10. Limestone, black, dense, no fossils. 11. Limestone, grey-black, very fossiliferous, porous. 12. Limestone, dark grey, very fossiliferous, dense, some breccia zones. 13. Limestone and shale, black to grey, a few fossils, breccia. 14. Shale, black, platy, siliceous, nonfossiliferous. 15. Shale, black, platy, siliceous, nonfossiliferous. ou don a. On the location map, Figure 1.28, construct a lithofacies map of this region. Label the fore-reef, reef, and back-reef areas. (See the Permian Reef cross section, Fig. 1.27) b. In which direction was the shoreline (land) during this interval of the Permian? c. Contrast the water conditions that probably existed in the northwest corner of the map area with those in the southeastern corner d. Where would you make your speculative land purchase? (Show the location on the litho- facies map.) What were your reasons for choosing this area?

Answer 1

To find the cavernous limestone bed, the first step is to mark out the contacts and draw an approximate geological map. Since we do not know the strikes and dips of the bed, we can only construct an approximate map.

This has been done using the data given. The yellow points in the map represent anhydrite, the maroon points represent grey shale, the green points limestone and the orange points black shale.

The contacts will pass bettween two groups of points, but we do not know the exact trend, just a general direction. The orange line is drawn between limestone and black shale, the green line between limestone and grey shale, the maroon line between grey shale and anhydrite. The map is given in the image below:

LOCATION OF NEW CAVERN OPENING FIGURE 1.28 Mop view of sample data locations

b) Anhydrite is a evaporite deposit formed when the rate of evaporation is greater thanthe rate of water inflow. This is possible only close to the shore, and thus the shore in this region was present towards the northwestern corner.

c)As mentioned earlier, the shoreline existed close to the northwestern part of the map and thus, the water level here was shallow. Limestone and shale are formed in deeper waters. As these are in the south eastern part, it is clear that water depth was least in the northwestern corner and increased towards the southeast.

d. Caverns are typical structures found in limestone terrains. Thus, if we are looking for caverns on our land, we need to by the part covered in limestone. It is suggested to by the entire part between the green and orange line towards the southeast which is all the limestone on the map.to increase your chances of finding the caverns