STOP V-1. Muleshoe bioherm. End of old ranch road and mouth of Muleshoe Canyon. Begin approach to Muleshoe bioherm from here on foot. At the start of the walk, we may observe a number of features. The lowest dark cliff represents the lower part of the Montoya Formation; the higher and thinner, resistant, dark-colored rocks are the partly silicified Fusselman dolomites (Silurian). Considerably higher up the mountain-side are the lighter-colored cliffs of the Lake Valley Formation (mainly Tierra Blanca and Doña Ana Members). The major cliffs farther back are formed by the nearly 300 m (1,000 ft) section of the Pennsylvanian Bug Scuffle Limestone Member of the Gobbler Fm.

Looking at the Muleshoe bioherm itself (Fig. 36), note the arched appearance of eroded flank beds on the southwest side of the mound. Below this arch is exposed a small portion of core facies that will be our final objective. From the entrance to Muleshoe Canyon, the route passes up the wash about 200 meters (650 ft), then begins to climb out of the wash on the north side of the canyon, angling eastward toward a point that will bring us to the same elevation as the base of the bioherm but several hundred meters east of it and standing on non-biohermal sediments (Photo Fig. 41). The approach to the bioherm climbs out of the wash over Silurian and Devonian rocks of the Fusselman, Onate, Sly Gap and Percha(?) Formations. About 20 m (60 ft) of Mississippian Caballero (or Andrecito) Formation unconformably overlies the Devonian and underlies the biohermal Lake Valley Formation. On reaching an elevation equal to that of the base of the bioherm escarpment and a position about one-third of a mile east of the bioherm, we can observe typical inter-bioherm lithologies of the Lake Valley Formation. From this point, a westward traverse along the base of the bioherm leads us from normal horizontally-bedded Lake Valley sediments into increasingly steeply dipping biohermal flank beds, blocks of core rubble, and finally into bioherm core facies beneath the ³arch² viewed earlier from below.

The bioherm has been studied in increasing detail for over 40 years (Ahr, 1989; Bowsher, 1948; Jackson and DeKeyser, 1984a; Jackson and DeKeyser, 1984b; Lane, 1982; Lane and Ormiston, 1982; Laudon and Bowsher, 1941; Laudon and Bowsher, 1949; Meyers, 1974; Pray, 1958a; Pray, 1975; Shinn and others, 1983). General descriptions were first given by Laudon and Bowsher (1941; 1949), who subdivided the Lake Valley Formation into six members. Muleshoe bioherm occurs in the second, third and forth members from the base of the Lake Valley, rises above the last two members of the Lake Valley and above the level of the overlying Rancheria Formation and protrudes into the base of overlying Pennsylvanian deposits of the Gobbler Formation (Pray, 1958b; 1961). Bioherms about 8 km (5 mi) to the north are decidedly elongate in a north-south direction and are not as thick as Muleshoe bioherm. Muleshoe is estimated to have stood more than 100 m (300 ft) above the general sea floor and may have developed on a relatively deep portion of a shelf that became more shallow to the north. Armstrong (1962, 1967) has suggested that a starved basin lay to the south, and tidal flat deposits are found 260 km (160 mi) to the north. Land formed by the Pedernal Uplift lay about 130 km (80 mi) northeast. Muleshoe bioherm appears almost circular in plan and may have formed in deeper water than the bioherms to the north. Wilson (1975a) stated that most geologists acquainted with Mississippian bioherms believe they accumulated below wave base and perhaps below the photic zone. Again, it is tempting to make comparisons between these bioherms and the lithoherms of the Straits of Florida described by Neumann et al. (1977).

Ahr (1989) examined the regional setting of the Mississippian bioherms and concluded that they formed on a very gentle ramp. The bioherms were ³preferentially sited on paleobathymetric highs of tectonic and depositional origin² Ahr (1989, p. 211). These structural and depositional features were very subtle, however, with only minor topographic relief.

It is interesting to note that the faunal components of the bioherm did not require light for their survival. The common forms we will see in both the core and flanking beds are crinoids, brachiopods, bryozoans, and solitary corals. In contrast to other bioherms observed on this trip, calcareous algae are absent. Other typically shallow-water forms, such as massive corals, clams and calcareous sponges, are rare or absent. The crinoidal grainstones of the flank beds (Photo Fig. 42) are poorly sorted and contain articulated columnals several inches long, which suggest flanking crinoidal beds formed in close proximity to where the crinoids lived. These biogenic sediments appear not to have been transported far from their source. As we approach the core facies of the bioherm, the slope of these flank beds increases to nearly 40 degrees.

The core facies has been studied in great detail by Pray (1958a; 1965a; 1965b; 1969), Lohmann and Meyers (1977), and Shinn and others (1983). Approximately two-thirds of the core consists of mud and the major faunal component of the bioherm cores is fenestrate bryozoans (Photo Fig. 42). There is considerable question as to the origin of the carbonate mud in the Muleshoe bioherm and other Paleozoic mud mounds. Are the muddy cores the result of currents piling up fine-grained sediment? The circular plan view of some mounds would make this possibility unlikely. Some authors (Dix and James, 1987; Tsien, 1985) consider the mud to have been at least partly contributed through the activities of simple bacteria and/or algae (perhaps negating the arguments about water depth). Other authors (McKinney and others, 1987), have suggested that fenestrate bryozoan colonies may have produced strong cillia-generated currents which led to trapping and accumulation of mud produced from a variety of biogenic or abiogenic sources.

Throughout the core facies, the sediments contain bryozoan fragments coated with banded, isopachous cement, which is cloudy when viewed in thin section (Photo Fig. 43). This cement was interpreted as marine in origin by Pray (1965a; 1965b). The cloudy cements contain inclusions of microdolomite, as illustrated by Lohmann and Meyers (1977), and are believed to be diagenetically altered high-Mg calcite marine cement. The presence of this penecontemporaneous, generally stable cement has precluded much compaction in the bioherm core facies, unlike its flank equivalents (Shinn and Robbin, 1983).

The details of the construction of Muleshoe bioherm and similar Lower Carboniferous mud mounds still hold many questions. What was the source of the mud? Why are the flank beds so distinct and sharply separate from the core facies? What role did submarine cement play in building these mounds? What localized the position of such mounds? Why are they so abundant in the Upper Paleozoic and so scarce in all other intervals? Many of these questions cannot be answered very satisfactorily. Volumetrically, cement is not nearly as important in the core facies of Muleshoe bioherm as it is in the Laborcita lithoherms. Submarine cement by itself did not build Muleshoe bioherm, but it would take only a small amount to act as a binding agent to hold the core facies together. If this were case, then the next question is why did the cement form here, localized in this mound? The sandy flank beds evoke clear visions of crinoidal meadows on the sides of the bioherm, but why not on the top or in the center, where the core facies predominates? Was the central position of the bioherm dominated by some mud-producing organism that decayed so completely that no trace is left behind? Or, was the center isolated from nutrient-rich currents, which fed animals on the sides of the bioherm? Wilson (1975, p. 165-167) suggests formation through a combination of hydrologic accumulation and baffling by crinoids and Bryozoa to form the muddy core facies. Gentle currents winnowed the sides of the bioherm.

Again, it is interesting to draw comparisons between these mounds and modern lithoherms in the Straits of Florida (Neumann and others, 1977). The processes involved in their formation include (1) hydrologic accumulation, (2) biogenic sediment contribution and biologic entrapment of sediment, and (3) subsea lithification by marine cement. An organic framework, typical in modern reefs, is absent in these Mississippian mounds and is not a requirement for mound growth. Although organisms and hydrologic regime are not directly comparable between modern lithoherms and Muleshoe bioherm, a combination of the three processes active in modern lithoherms could probably account for the features we see in these Mississippian buildups.

The diagenesis in non-biohermal Lake Valley sediments has been studied in considerable detail during the last 10 years. Using cathodoluminescent petrography, Meyers (1974) found five generations of cement in crinoidal grainstones like those we observe off the flanks of Muleshoe bioherm. Most of this cement occurs as syntaxial overgrowths on crinoidal sands. Marine cements like those in the bioherm core facies are rare in inter-bioherm areas. The syntaxial, clear overgrowths are interpreted to have formed in a fresh-water aquifer that occupied Lake Valley sediments during periods of sea level change and regional subaerial exposure. Careful examination of cements at post-Lake Valley unconformities revealed three cement zones to be pre-Rancheria Formation and two more to be post-Rancheria but pre-Gobbler deposition. These two periods of subaerial exposure led to a great porosity loss in these sediments and resulted in 90-95% of the total intergranular cement in Lake Valley grainstones (Meyers, 1978).

From outcrop of core facies beneath the Muleshoe ³arch,² hike back downslope to vehicles.

OPTIONAL STOP. Retrace route east along the base of the bioherm escarpment about 150 meters (500 feet) to a prominent gully. Cross gully and scramble up dipslope, angling westward to top of bioherm. CAUTION! The route is considerably more steep and more difficult than that below. Use careful judgement in making this climb. Excellent exposures of steep flank beds are crossed during the ascent and the exposure gives one a real ³feel² for the original depositional slope. On top there are well exposed examples of core mudstones, submarine cements and clastic dikes. The best exposures are above the cliffs in the southwest side of the bioherm. Cross the top of the bioherm to the north side for excellent view of the upper Lake Valley members lapping the sides of the bioherm and the unconformable relationships between the Lake Valley, Rancheria and Gobbler Formations. A descent can be made down the gully between the bioherm and surrounding strata of the north side. Contour around the base of the bioherm to the southwest side, and then descend the slope to Muleshoe Canyon and back to the vehicles.

Retrace route back to U. S. 54.

Left turn to entrance of El Paso airport. El Paso, the "Sun City", lies at an elevation of 1,130 to 1,220 m (3,700-4,000 ft) and has an average annual rainfall of just 19.86 cm (7.82 in). The area was first visited by early Spanish explorers in the 1500¹s and a mission was established here in 1659 by the Franciscans. By the mid to late 1700¹s the region (El Paso del Norte) was an established trading center and stopover for caravans. In 1848 El Paso (then known as Franklin) became part of the U.S. after the Mexican War was settled by treaty. Since then the town, with its sister city of Juarez, Mexico, has become a major center for commerce, especially manufacturing of clothing, leather goods, and other labor-intensive articles. Smelting and refining also are major activities (the ASARCO smelter has one of the tallest smokestacks in the nation). The town is also the location of the University of Texas at El Paso campus, Fort Bliss and the National Air Defense School (a major site of NATO pilot training). El Paso has more than 550,000 inhabitants and Juarez has a population which is now well over 1.5 million persons. These figures make El Paso the 4th largest city in Texas and the 29th largest in the U. S.; Juarez is the 4th largest city in Mexico.

If time is available, we will examine the Cambro-Ordovician section on Scenic Drive (I-10 to Piedras Street exit; north on Piedras to Richmond; left on Richmond to Scenic Drive). Alternatively, we can examine the igneous and metamorphic (including metamedimentary) rocks on theTrans-Mountain Highway (which directly intersects U. S. Hwy. 54).

END OF ROADLOG.