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Isotopic and Elemental Evidence For Meteoric Alteration of A Pennsylvanian Phylloid-Algal Mound, Holder Formation, New Mexico, U.S.A.
Yucca Mound, a Virgilian (late Pennsylvanian, late Kasimovian) phylloid-algal bioherm exposed in the Sacramento Mountains of south-central New Mexico, U.S.A., developed on a shallow shelf off the Pedernal landmass. The bioherm contains two distinct exposure surfaces, one within the unit and one capping it, identified by the presence of red-orange oxidized zones and pedogenic features. Through petrographic and geochemical analysis five low-Mg calcite cement phases have been identified that occlude most primary and secondary porosity.
A study of the cement phases associated with exposure surfaces indicates that this mound experienced two distinct episodes of meteoric diagenesis. As meteoric water initially passed through the bioherm, during relative sea-level fall and development of subaerial exposure surface 1, considerable porosity was created through the dissolution of aragonitic grains. Early, during this first period of exposure, Early Cement I was precipitated from mixed meteoric and marine waters, which also recrystallized fibrous high-Mg marine calcite cement. Early Cement I is characterized by a mottled cathodoluminescent microfabric and displays a higher Mg content and more positive δ18O values than Late Cement I, which formed as sea level continued to fall. Late Cement I, interpreted to have formed from meteoric water, has an average δ18OVPDB = −4.8‰. Relative sea level then rose and marine waters once again inundated the mound and flanking facies, leading to renewed deposition. A second episode of subaerial exposure is manifested by development of a soil zone and precipitation of Cement II and Cement III (δ18OVPDB = −5.6‰). Cement II and III are followed by luminescent Cement IV, which displays an average δ18OVPDB = −6.3‰, which indicates that this cement phase most likely precipitated in an increasingly reducing system, under shallow burial, from interstitial waters that remained after the second stage of meteoric diagenesis. Remaining porosity was occluded by Cement V (δ18OVPDB = −7.3‰), which was precipitated from connate waters during a later stage of burial.
This study confirms the presence of multiple periods of subaerial exposure for Yucca Mound and clearly demonstrates distinct meteoric signatures for each exposure event, evidenced in both the geochemical and the petrographic characteristics associated with each phase of cementation and diagenetic alteration. This demonstrates that through detailed geochemical and petrographic analysis of the diagenetic products it is possible to unravel the complex diagenetic histories of deep-time carbonate sequences.
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