New Mexico Geological Society Annual Spring Meeting — Abstracts

Pennsylvanian transgressive-regressive (T-R) cycles may be produced by eustasy (including glacioeustasy), autocyclic deltalobe progradation and avulsion, or tectonism. Detailed study of a 300 m section of Middle and Late Pennsylvanian Madera Formation in the Sandia Mountains suggests that some T-R cycles require eustasy and can be recognized from study of a single section. A 120 m interval contains 15 T-R cycles; the implied high frequency of fluctuation suggests eustatic or autogenic, not tectonic, controls. Regressive and transgressive depositional sequences are each divisible into two types. R1: delta-plain clastics rest on erosion surfaces cut on sub-tidal limestones. Limestone facies
typically record abrupt shoaling within a meter of the contact with overlying clastics. Subaerial exposure is commonly indicated, but there are no inter-tidal or supra-tidal facies. Meteoric calcite cement increases and compaction decreases upward through carbonates. R2: gradational transition from limestone to prodelta and delta-front (±delta-plain) facies. T1: abrupt transition from delta-plain clastics to subtidal carbonate, with common phosphatic lags on the ravinement surface. T2: transition from delta-plain to marine clastics that grade into sub-tidal limestone. R1 sequences require rapid sea-level fall; R2 sequences could be autocyclic or eustatic. T1 sequences are typical of abrupt facies transitions produced by shoreface retreat and the Tilechanism for transgression is not clear. T2 sequences record transgress ion in the presence of abundant sediment supply and adequate eccomrnodation to preserve transgressive clastics; these characteristics are favored by rapid sea-level rise without immediate delta-front retreat. Detailed field and petrographic study of single sections can be used to evaluate eustatic signals, especially with regard to the nature of the regressive hemicycles.