CARLSBAD-ALAMOGORDO (NEW MEXICO)
GEOLOGIC ROAD LOG

STOP IV-2. Virgil bioherm. Lincoln National Forest boundary sign and parking area on left. The west flank of the bioherm is strikingly exposed to the north of the road. Beds with apparent dips of almost 45° at the west end give way to less steeply dipping strata and massive units in the center of the bioherm outcrop. Faint bedding near the center suggests the escarpment does not expose core facies or that the bioherm core is very similar to flanking beds. The 18-25 m (60-80 ft) thick feature is typical of many such bioherms in the area and shares features in common with many late Paleozoic buildups.

Plumley and Graves (1953) first described the Virgilian bioherms and emphasized their geometry, orientation and biological origin. They appear as elongate bodies up to 1.6 km (1 mi) long and 60 m (200 ft) thick (Photo Fig. 36). They tend to parallel the mountain front. As we traverse the outcrop here, it will become apparent that the bioherm, while beautifully exposed, does not weather in a manner that allows easy interpretation of the composition of these limestones. Parks (1958, 1962), Wray (1959, 1963) and Konishi and Wray (1961) established the importance of platy algae in these limestones and have refined the taxonomy of the platy or phylloid (leaf-shaped) algae (Pray and Wray, 1963). Phylloid algal limestones are widespread in the United States (Wray, 1968), and bioherms composed of such algal limestones are widespread in Late Pennsylvanian and Wolfcampian strata of southern New Mexico (Wilson, 1977).

Cline (1959) emphasized the cyclic nature of Virgilian rocks in this area, and Wilson (1967) related these shelf cycles to basinal cycles in the Orogrande basin. Shelf cycles consist of a variable sandstone and shale lower member with local channel-fill conglomerates which grade upward into normal marine limestone and shale. These in turn pass upward into shallow-water limestones (grainstones and bioherms) which cap the shelf cycles. Wilson (1967) interpreted these cycles to be the result of repeated sea level fluctuations, which periodically exposed the shelf and bioherms to subaerial weathering and diagenesis. Wilson drew on conceptual models of cyclic deposition and evidence from the Holder Fm. to develop the idea of shelf and basin reciprocal sedimentation. This important model promotes alternate sites of basin and shelf sedimentation during sea level low and high stands, respectively. During lowered sea level, most sediment bypasses the exposed shelves and is deposited in adjacent basins. During sea level high stands, these sediments are deposited on the shelves along with shallow-water limestones, while the basins receive little sediment and are ³starved.² The reciprocal sedimentation concept has been widely applied in shelf/basin sediment dynamics, and we will discuss its application to the Permian basin.

Pray (1961) defined the La Luz anticline, which runs approximately NNW through this area and had a pronounced local influence on Late Pennsylvanian sedimentation (Wilson, 1969). Bioherms developed with long axes approximately parallel to the axis of the La Luz anticline, which was a subtle structural feature at the time but developed more strongly during later deposition of the Holder Formation.

Inspection of the bioherm proceeds up a gully around the west end of the feature (Photo Fig. 37), upward across flanking beds and then eastward to the top of the mound. Before reaching the mound proper, the climb traverses marine shales and limestones of the upper Beeman Formation and enters the Holder Formation approximately 18 m (60 ft) below the base of the bioherm. The limestones include transported oolitic and algal grainstones and algal boundstones separated by slope-forming shales. Note the lenticular (channelized) and sometimes graded bedding of these flank debris units. In places, this debris is draped over small (1-5 m; 3-15 ft) satellite mounds which grew in deeper water areas skirting the fringes of the main phylloid algal mounds. The satellite mounds are dark-colored, have knobby or finger-like microstructure, are domal (sometimes with an atoll-like structure). In thin section, these mounds are seen to be complex intergrowths of plumose (blue-green?) algae, encrusting Foraminifera, and minor marine cements (sometimes with relict aragonite crystal inclusions).

The bioherm is composed in large part of mud (micrite) and phylloid algal plates. The phylloid algae are usually poorly preserved as molds or replaced by blocky calcite with all traces of original microstructure lost. They are probably of diverse origin, some being green calcareous algae and some being red calcareous algae. Their poor state of preservation suggests they were originally aragonitic. Wray (1975, 1977) has suggested that the closest living analogues to some of the phylloid algae may be a family of red calcareous algae known as the Squamariacean algae. Today, ³squamies² are subtle but widespread coral reef inhabitants.

A great many other organisms are evident in the biohermal rocks, including stromatolites, sponges, tubular encrusting Foraminifera, stromatoporoids, and corals, all of which are capable of producing reef structures, but none of which appear to be abundant enough on outcrop to account for the bulk of the carbonate buildup. Volumetrically, mud is the most important constituent of the bioherm. Wray (1959) suggested that the mud had been trapped by a thicket of phylloid algae, probably in a relatively low-energy setting. Ball et al. (1977) questioned the mound-building capabilities of phylloid algae. Parks (1977b) briefly reported that algal plate mudstones were uncommon in four cores taken through the bioherm. The upper bioherm contained calcirudites of sponge, stromatoporoid, tubular Foraminifera, and other clasts. The lower part of the mound contained more mud and calcarenite and rare-to-abundant masses of fibrous calcite.

Earlier outcrop studies by Otte and Parks (1963) suggest that 30-50% of the lowest third of the bioherm is composed of botryoidal fibrous calcite, a replacement after aragonite submarine cement. The material is beautifully illustrated by the authors and was interpreted as fossil remains of Stromatactis-like organisms, following similar interpretations of fibrous cements from Europe. Otte and Parks (1963) point out that fibrous calcites weather indistinctly in the Virgil reef and are difficult to observe in outcrop, but are strikingly accentuated by weathering at Stop IV-4.

The lower third of the bioherm is also vuggy in outcrop with irregular voids up to several inches across scattered throughout the rocks. The origin of these vugs is problematic. Parks (Parks, 1977a) considered several mechanisms for producing the vugs, including subaerial solution (Wilson, 1975a), submarine solution, decay of pre-existing soft-bodied organisms, sheltered porosity, dewatering contraction and gas bubbles. He concluded that vugs were the product of a combination of decay and gas generation.

rtions of the bioherm and are evidence of early lithification. Wilson (1975a) suggests this early cementation and vuggy leaching took place during a sea level low stand and are the result of early meteoric water diagenesis. This explanation seems particularly likely in light of the cyclic nature of Holder sediments overlying the bioherm and the transgression/regression model that explains their origin (Wilson, 1967). More recent and more geochemically oriented work (Goldstein, 1988a; Goldstein, 1988b) has confirmed the repeated, complex nature of subaerial exposure effects in these strata. Sea level changes were of sufficient magnitude that meteoric waters penetrated through deposits of multiple prior cycles. Under such conditions, the degree and nature of diagenetic alterations depended on timing of exposure events, presence or absence of overlying aquicludes, and diagenetic susceptibility of the sediments.

The abundant evidence of submarine cement illustrated by Otte and Parks (1963) and Parks (1977a) suggests that early submarine cement should be considered as the lithifying agent in these buildups. Less obvious micritic submarine cements may also be present in the bioherm sediments, cements which appear identical to detrital micrite. Such cements are common in Holocene reefs, are composed of high-Mg calcite, and appear as a micritic matrix in the reef rock (Macintyre, 1977). It is intriguing to imagine what role submarine cement may have had in creating vuggy porosity in these reefs. In a sense, portions of the bioherm are really ³lithoherms,² following the terminology used by Neumann et al. (1977) to emphasize the role of subsea cementation during growth of the structure. This role is nicely illustrated by substituting submarine cement for ³Stromatactis² and quoting from Otte and Parks (1963), ³(Submarine cement) functioned as both a sediment-binding and a framework-building organism in the construction of the bioherms and may be quite widespread in other late Paleozoic bioherms of the western United States.²

This has proven to be a rather prophetic statement in light of the now widely recognized submarine cements present in reefs of all ages.

STOP IV-3. Yucca mound. An unused dirt trail leads north of U. S. 82 about 300 m (1,000 ft) until it intersects the gully to the right (east). Route turns up gully at intersection until Yucca Mound is reached, about 100 m (300 ft) after leaving the trail. The beds exposed in the gully below the mound and the mound itself have been studied in considerable detail by Toomey et al. (1977a; 1977b).

Exposed in the gully bottom are beds interpreted to represent facies which are basinward and slightly older than the mounds. These include shales, some of which show evidence of penecontemporaneous deformation (slumping?), crossbedded grainstones composed of material presumably transported from mound areas (e.g., sands composed almost entirely of fragments of tubular Foraminifera), and small satellite or precursor mounds (3.5-4.5 m; 12-15 ft in diameter) composed of plumose algae and Foraminifera.

The core facies of the mound is well exposed and consists of mud and algal plates, and a great variety of other fossils, including sponges, Foraminifera, pelecypods, and dasyclad (calcareous green) algae.

Mound geometry can be seen by climbing the steep slope on the north side of the gully and looking south across the canyon that transects the mound (Photo Fig. 38). Beds immediately above the mound are nearly horizontal on top of and east of the buildup, but dip steeply over the western edge.

The bioherm is a complex of two mounds, as pointed out by Toomey et al.(1977a), and in detail includes a complex variety of carbonate lithologies. One mound overlies and is basinward of the other, showing that the complex as a whole built seaward and is regressive in character. However, the presence of stratigraphic breaks in the complex, interpreted to be a subaerial origin, emphasize the complex history which gave rise to this generally regressive sequence. Toomey et al. (1977a) estimate the position of the mound to be 0.8 km (1/2 mi) east of the shelf edge. The shelf at this point was narrow (a few kilometers?) with the Orogrande basin to the west and the Pedernal Uplift to the east.

Submarine cements have not been identified in the main Yucca mound and appear not to have been important in mound development here.

STOP IV-4. Laborcita lithoherms (sometimes termed Scorpion mound). Hilltops 0.4 km (1/4 mi) to the NE are capped by strikingly banded limestones of Wolfcampian age (Photo Fig. 39). From this distance, it can be seen that the dark bands (cement-rich) pinch and swell along the face of the outcrop. In contrast, the light bands (mud-rich) are of rather even thickness and drape over the topography of the dark bands. This is strong evidence for the excellent mound-building capabilities of cement.

Walk to the lithoherms via a meandering route over beds of the Laborcita Formation of Otte (1959a; 1959b). Here, these beds (partial time-equivalents of the Abo and Hueco redbeds) include red and green sandstones, siltstones and mudstones with some spectacular polymict conglomerates and thin limestones. At a few horizons, one can see superb oncolites (Toomey, 1983) < brachiopods, cephalopods, and other grains concentrically coated by the red alga Archaeolithophyllum lamellosum Wray. These deposits represent very shallow-water marine sedimentation, probably in water depths of only about a meter (Toomey, 1983).

The overlying lithoherms have been described in increasing detail by Otte (1954; 1959a; 1959b), Otte and Parks (1963), Cys and Mazzullo (1977), Mazzullo and Cys (1979), and Shinn et al. (1983). Because of their proximity to underlying nonmarine beds, the lithoherms are thought to be nearshore marine buildups, perhaps analogous to fringing reefs. Otte (1959a) estimates the lithoherms to have stood as much as 20 m (60 ft) above the surrounding bottom. Again, we use the term ³lithoherm² (Neumann et al., 1977) to describe these mounds in order to emphasize the inferred role of submarine cement in mound development.

Weathering of these mounds has left internal structure easily visible. A number of components may be readily recognized in outcrop (Photo Fig. 40). These include: (1) gray, commonly well laminated lime mud; (2) dark calcite cement, which fractures to reveal a blocky structure but may be viewed in low angle light to reveal a relict fibrous habit; (3) phylloid algal fragments; (4) fractures; (5) scattered sand pockets, some graded; (6) some coarsely crystalline, white, pore-filling calcite, and (7) brown dolomite. The dark calcite cement derives its color from submicroscopic inclusions of organic matter, which is apparently not extractable (Plumley and Graves, 1953). On outcrop, some areas of the lithoherms may be seen to be extremely rich in this dark cement. In places, it forms an anastomosing network with lighter-colored sediments infilling voids within the cement framework. Cys and Mazzullo (1977) and Mazzullo and Cys (1979) estimate that this cement accounts for 50-85% of the mound volume. They interpret the cement to be a marine precipitate that grew on the sea floor and within voids in the mound. It is interesting to compare these figures with estimates of 20-40% in-place coral in modern and Pleistocene coral reefs. It would appear that there is considerably more cement framework in the Laborcita lithoherms than there is coral framework in many coral reefs.

At several locations along the exposure of the lithoherm, the contact between a dark, cement-rich layer and a lighter, mud-rich layer may be observed in considerable detail at close range. Note that this contact is not erosional (and that the relief on the cement-rich bands is not erosional), but rather the contact appears to be a sharp change in the character of sedimentation. It is quite clear from these outcrop relationships that it is the marine cement in these mounds that has been responsible for their reef-like growth. These exposures provide the best evidence, in the writers¹ opinions, of ancient examples of lithoherms, and they are certainly the most extreme case of submarine cementation in mounds that we will observe on this trip.

A short core through this mound, taken by the U. S. Geological Survey in the early 1980¹s (Shinn et al., 1983), indicated that these mounds were localized on ³thicks² in the underlying sandstone beds and that these mounds were extensively brecciated during burial rather than because of subaerial exposure. The contrast between the highly cemented layers and the mud-filled zones led, in their opinion, to intense grain breakage as a consequence of overburden stresses. Other authors (e.g. Dunham, 1969), however, have examined age-equivalent buildups and concluded that brecciation was due to meteoric dissolution and collapse.

Turn around and retrace route backwards to junction of U. S. 54-70 with U. S. 82.

Junction of U. S. 54-70 with 10th Street, Alamogordo, N.M. Alamogordo is a town of some 35,000 persons. It provides support for another 7,500 persons at the nearby Holloman Air Force Base. Rainfall in the Tularosa basin averages about 25 cm (10 in) per year at the roughly 1,300 m (4,300 ft) elevation of the town but gets as high as 122 cm (48 in) per year in the nearby mountains.

Although minor early Indian and Spanish settlements existed in this region, the town was really established as a model planned community in 1898 when the railroad from El Paso reached here. Establishment of a large Army training base during World War II led to major growth of the town. The area was chosen for the first atomic bomb test — at the Trinity site which lies about 105 km (65 mi) northwest of Alamogordo, at the northern end of the 350 year old Spanish Jornado del Muerto (Journey of Death) trail. Expanded rocket research after the war at the White Sands Missile Range led to the growth of the town to its present size.

White Sands National Monument lies about 24 km (15 mi) southwest of Alamogordo on U. S. 70-82 and is well woth visiting. These dunes are virtually pure gypsum which has accumulated in the closed drainage of the Tularosa basin after being leached from Permian (mainly Yeso-San Andres) evaporites in surrounding mountain ranges. The gypsum sands are produced at Lake Lucero through evaporation of near-surface groundwater and are then blown eastward into these dunes. For details of the deposition of these evaporite sands see references listed on White Sands National Monument page.

Roadlog ends.