Abstract

Detailed analysis of measured sections within the Madera Limestone in the Sandia Mountains, New Mexico characterize depositional environments and transgressive-regressive (T-R) cycles across a west-dipping ramp situated approximately 70 km west of the Pedernal Uplift, a tectonically active element of the Ancestral Rocky Mountains. The ramp setting was separated from the Pedernal Uplift by the deep, clastic-filled Estancia Basin.

T-R cycles are 2-12 m thick and consist of asymmetric, shoaling upward calcareous shales, thick skeletal wacke-stones and packstones-grainstones, and are capped by subaerial exposure surfaces or terrigenous elastics. Delta-plain elastics rest on subtidal carbonates in landward sections and correlate basinward with thin, transgressively reworked sheet sandstones. The presence of subaerial exposure features upon subtidal carbonates argues for an allogenic control on cycle formation.

15 T-R cycles occur between fusulinid zones of B. arizonensis and B. girtyi and are interpreted to coincide with “3rd-order” eustatic curves of Ross and Ross (1987). Following the convention of most workers, these “3rd-order” cycles are herein referred to as 4th-order. No higher order cycles have been recognized. Cycles possess a bundling of four 4th-order cycles into larger, asymmetric 3rd-order cycles, similar in pattern to those described by Ross and Ross (1987). Correlation of Madera Limestone cycles with those in other regions indicates that Ancestral Rocky Mountain tectonism was less effective than eustasy in controlling the stratigraphy of Middle Pennsylvanian sediments.

Variations in facies and cycle thicknesses across depositional strike reflect morphologic and bathymetric irregularities parallel to shoreline. The greater abundance of grainy sediments in certain sections relative to less grainy sections and the persistence of deltaic activity in localized areas also indicates the presence of local topographic highs and lows along the ramp.

Using various time scales for the Desmoinesian, cycle periodicities range between 200,000 to 473,000 years. This average cycle periodicity is similar to other 4th-order cycles reported in the literature for the Pennsylvanian and suggests a Milankovitch orbital forcing mechanism.