Fluctuations in Late Pennsylvanian (Virgilian) seawater chemistry inferred from submarine cements of phylloid algal mounds, western Orogrande Basin (New Mexico)
— Shane C. Seals, Gerilyn S. Soreghan, and R. D. Elmore

Reworked clasts of submarine cements exhibiting fine fabric preservation occur within a thin (20 cm), shale-encased lithoclastic packstone present in the proximal mound flank of a phylloid algal mound complex from the western Orogrande basin (New Mexico). Mounds within this complex contain abundant recrystallized submarine cement in mound-core cementstone and foraminiferal/algal boundstone facies. Volumetrically abundant cementstone is particularly prevalent at the bases of high-frequency (glacioeustatic) sequences, while foraminiferal/ algal boundstone typically occurs at the tops of sequences. Although in situ submarine cement within the mound core is uniformly recrystallized, clasts of finely preserved (radial-fibrous) submarine cement occur within a lithoclastic packstone unit adjacent to the mound core. Here, we report on the optical and chemical properties of both in situ, recrystallized submarine cement from mound-core units and transported clasts of fabric-retentive cement preserved within the mound-flank unit.

Three cement types are observed as clasts within the lithoclastic packstone unit. Type 1 cement characterizes clasts derived from nonfossiliferous cementstone. It is radial-fibrous, light brown, and exhibits sweeping extinction. Solid inclusions of magnetite and/or pyrrhotite, and hematite line radial growth fronts within this cement type. Microprobe analysis indicates overall low Fe, Mn, and Mg and moderately elevated Sr with locally high Fe and Mn. Type 2 cement occurs within clasts of foraminiferai/algal boundstones; it is also radial-fibrous, light brown, and exhibits abundant sweeping extinction. In contrast to the type l cement, however, this cement lacks Fe-rich growth bands, and exhibits zones of fluctuating Mg concentration. Low-Mg zones 1000 ppm) contain low Fe and Mn and high Sr. High-Mg zones (> 4000 ppm) exhibit moderate Fe, variable Mn, and relatively depleted Sr. Type 3 cement consists entirely of recrystallized, sparry calcite fans which are colorless to light brown, non-pleochroic, and contain low Fe, Mn, and Mg and variable Sr. Type 1 and 2 cements occur only within transported clasts of the studied lithoclastic packstone, whereas type 3 cement occurs as clasts within the packstone and within basal-sequence cementstones of the mound core. 

Optical properties and trace element chemistry of all the cement types indicate pseudomorphism of an aragonite precursor. We infer that type I cement was derived from in situ cementstone formed in basal sequence positions within the mound core. Solid inclusions of Fe-rich phases within this cement together with the lack of biota suggest precipitation under oxygen-poor conditions likely associated with onset of glacioeustatic transgressions. In contrast, abundant biota and lack of Fe-rich inclusions in the type-2 cement of foraminiferal/algal boundstone indicates oxic conditions, These data suggest that seawater chemistry within the Late Pennsylvanian Orogrande basin evolved from oxygenpoor during early transgressions to oxygen-rich at highstands through glacioeustatic cycles.