Fluctuations in Late Pennsylvanian (Virgilian) seawater chemistry inferred from submarine cements of phylloid algal mounds, western Orogrande Basin (New Mexico)

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.