New Mexico Geological Society Annual Spring Meeting — Abstracts

Initial sedimentological studies of two correlative Pennsylvanian (lower-ta-mid Demoinesian) sections, located 4.3 kilometers apart, indicate two distinct depositional environments (within a ramp setting). Both sections, originally measured by Myers (1987), are approximately 140 m thick and occur within the established fusulinid faunal zones of B. arizonensis and B. haworthi. Both sections exhibit high-frequency asymmetrical cycles that may be interpreted as 3rd-order and possibly even 4th-order cycles. Correlation of cycles between the two sections is difficult due to the dramatic lithological differences between each section.

The northern section is dominated by cyclic, lower-to-mid subtidal mudstones and wacke/packstones with diverse faunal assemblages. Deeper water lithologies are characterized by dark, wavy, interbedded mudstones and calcareous shales that are rich in bryzoans. These immediately pass upward into cliff-forming, lighter colored, phylloid-algal wackestones and crinoidal-rich packstones/grainstones with only minor terrigenous clastic material. Four major cliff-forming cycles are represented in this section and may represent the four, 3rd-order cycles that are recognized globally in this part of the Demoinesian section (Ross and Ross, 1987),

The southern section contains a mixture of limestone and clastics and is dominated by upper-subtidal sandstones with oolitic-calcite coatings on the sand grains. Closer inspection of many of the sandy intervals have revealed medium-to-Iarge scale planar-tabular crossbedding, including varying degrees of bidirectional crossbedding, that may indicate tidally influenced currents in the area. Exposure surfaces marked by nonmarine facies and early meteoric cements in the oolitic sandstones may correlate to cycle tops in the northern, carbonate-dominated section.

Through the use of a third, middle section and from further analysis of the data collected, the following concerns will be addressed: (1) to determine the nature of lateral facies changes from a nearshore, tidally influenced clastic environment, to an offshore, carbonate dominated setting; (2) to develop a better understanding of the paleogeographic conditions that account for the abrupt change in facies associations; (3) to determine whether 4th-order (cyclothem) scale cycles can be readily identified and correlated to other Pennsylvanian I sections in the region; and (4) to determine the causes of cyclicity within the Los Pinos area.