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Summary

Lower Mississippian mounds in the Sacramento Mountains differ in tectonosedimentary setting, depositional environment, and biota from upper Mississippian (Asbian and Brigantian of Britain and Ireland) buildups in Texas, Utah, and Derbyshire, UK. Upper Mississippian mounds commonly occur on rimmed, or drop-off shelf margins and have shallow-shelf grainstones and packstones with taxonomically diverse skeletal biota as level-bottom equivalents (Ahr, 1989). Lower Mississippian buildups on the other hand are commonly found on distal parts of ramps in deeper-water settings (Ahr, 1989), and they are not associated with slope-breaks, shallow-water biota, or grainstone - packstone level-bottom beds with taxonomically diverse skeletal allochems. The Sacramento Mountain mounds and level-bottom beds are comparable in age and depositional setting to the European Waulsortian as described by Lees and Miller (1995) in Britain, Ireland, and Belgium, but It is difficult to make a detailed comparison for the following reasons: 1) data on the European buildups is limited to the mounds, rather than mounds plus contiguous level-bottom beds because of limited outcrop exposures in Europe, 2) information on mound constituents is presented in the form of relays or relay indices rather than as primary data, 3) the methods of data processing have varied in different studies, especially in correspondence analyses where the operator must select components without following a prescribed selection routine.

Age and Environmental Setting

European Waulsortian and Sacramento Mountain mounds and level-bottom beds occur on distal parts of ramps and are Tournaisian to Early Visean in age. Many European buildups are interpreted to have begun developing at water depths below the photic zone, which is defined on the presence-absence of fossil phototrophs and micritization (Lees and Miller, 1995). In this context, the shallowest Waulsortian phase, phase D, indicates an origin within the photic zone, and in some cases, above fair-weather wave base (autochthonous ooids). Fossil green algae and micritization are rare to absent in the Sacramento Mountain mounds and sediments, and “phase D” consitituents are essentially absent from all mounds.

The developmental history of the Nunn and Tierra Blanca mounds in the Sacramento Mountains is punctuated by hiatuses, but mounds in the Alamogordo Member represent comparatively continuous deposition. Nunn - Tierra Blanca mound lithology and biota may change across hiatuses, especially in the large mounds like Muleshoe and Sugarloaf. This pattern of interrupted growth is commonly attended by phase inversions or omissions, a characteristic that either is not common and important, or has not been recognized, in the European Waulsortian sections. Component assemblages (phases) in Alamogordo mounds do not regularly occur in predictable patterns such as phase A at mound base and phase C at mound top. A depth-dependent sequence of biotic assemblages does, however, exist in Alamogordo Member level-bottom beds (Jeffrey and Stanton, 1996), and may represent the closest parallel in New Mexico to the phase array described from European Waulsortian mounds.

Lithology and Biota

European and New Mexican buildup lithologies are similar, especially the Alamogordo Member-equivalent mounds and the European Waulsortian buildups. Nunn and Tierra Blanca-equivalent mounds appear to consist of much greater volumes of cement than their European counterparts, and they also appear to be marked by more numerous and much larger cavities, fissures, and neptunian dikes than the European buildups. The full significance of these internal cavities to the biotic makeup of the New Mexican mounds is not clear. Peloidal muds and “polymuds” (Lees and Miller, 1995) are volumetrically more abundant in the European buildups. Alamogordo Member mounds contain much more poorly-bedded micrite as neomorphic microspar, and Nunn - Tierra Blanca mounds tend to be cementstone buildups with peloidal and polymuds confined to cavity fillings.

European and New Mexican buildups, along with the Sacramento Mountains level-bottom beds, contain mainly the same taxa, however the New Mexico mound constituents do not occur in regular and continuous phases from mound base to top. As previously mentioned, the phase arrays commonly exhibit omissions, inversions, or repetition of phases on vertical transects through the mounds of all ages. To test for parallelism between New Mexican and European constituent arrays, several data processing methods were employed.

Data Processing

Petrographic data from the New Mexican mounds were first processed with an early version of the Jaccard similarity coefficient program developed by Hennebert and Lees (1985) to seek relays within Alamogordo mounds, younger mounds, and all mounds. Relays consist of assemblages in which some constituents occur in overlapping vertical ranges from one assemblage to the next, perhaps indicating a gradual shift in constituent composition that, in turn, represents a gradient in the paleoenvironment. The results were inconclusive at all stratigraphie levels because the calculated matrix of similarity coefficients did not reveal relays. Constituent assemblages from samples ranging from base to top of all mounds in the Alamogordo, and from the Nunn - Tierra Blanca-equivalent portions of Muleshoe Mound were grouped into phases based on key constituent types identified in the Waulsortian buildups of Europe by Lees and Miller (1985). This proved to be unsatisfactory as well, because the phase A-to-D progression of Lees and Miller (1985) could not be found in any of the Alamogordo or other mounds in the Sacramento Mountains. Instead, samples typically exhibited phase inversions such as phase C directly to phase A, or phase B to phase A, or vertical omissions such as phase A directly to phase C, even in Alamogordo samples taken from mounds without apparent discontinuities. The irregularities in phases exist throughout the mounds rather than on a sample-by-sample basis, and inversions may be interlayered with omissions, or vice-versa, suggesting that the absence of ordering in the vertical progression of phases is not due to missing section. There is, however, a general tendency for more phase C constituents to occur in the upper parts of all mounds.

Correspondence analysis (Hennebert and Lees, 1991) is used by Lees and Miller (1995) to produce a graphical array of constituents or samples such that the array is construed to represent an environmental gradient in a manner that parallels the relays of phases A through D in European Waulsortian buildups. Relay indices are constructed to indicate the parallelism (Lees and Miller, 1995). The Hennebert and Lees (1991) correspondence analysis program was run by Ahr on data from Alamogordo mounds, Nunn-Tierra Blanca mounds separately, and on data from all the Sacramento Mountain mounds combined. Runs were made with New Mexican data as subsets compared with 1988-vintage data on European Mounds given to Ahr by Alan Lees. Regardless of which constituents were chosen for program runs of New Mexico-only data, Alamogordo, as well as Nunn-Tierra Blanca mound constituent groups did not produce a correspondence “arch” like those illustrated by Lees and Miller (1995). Instead, when New Mexico data were run in correspondence analysis for comparison against the European data set of 1988, the New Mexico results produced a point cluster that fits in the central portion of the European array (“arch”). The significance of this result is not clear because of these equivocal results. Finally, unranked presence-absence data were run in the database program “Paradox for Windows” to search for patterns of occurrence in constituent composition, but the results did not reveal consistent patterns. Therefore, the analysis presented in this paper is based only on primary, non-ranked, presence-absence data.

Constituents from the level-bottom beds studied by Jefferys and Stanton (1996) occur in assemblages similar to the phases in European Waulsortian mounds described by Lees and Miller (1995). It appears that environmental gradients existed along the Sacramento Mountain ramp and that they influenced the biotic composition of the Alamogordo Member level-bottom sediments. These gradients probably represent continuous change along the sloping sea floor, leading to the conclusion that the parallel patterns seen in the European Waulsortian phases represent gradients -- probably depth, light, and agitation from inception to termination of buildup growth. The absence of regular progressions in component phases in Alamogordo mounds suggests that the small mounds did not develop enough bathymetric relief in deep water (below storm wave base and possibly below the photic zone) to have been influenced by environmental gradients. The absence of phase D components, the phase inversions and omissions, the hiatuses in mound growth, along with the massive production of fenestrate bryozoan cementstones in the Tierra Blanca equivalent mounds indicates that the larger mounds developed in a vigorous and variable hydrologie regime, but one that may not have been above fair weather wave base. Such hydrologie activity was probably related to thermohaline, geostrophic, or storm currents.

References

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References

See reference list with Introduction chapter to this guidebook

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