ABSTRACT: Transgressive vs. Regressive Phases as a Control on Early Diagenesis: Lisburne Group, North Slope, Alaska
Teresa A. Imm
Within the Mississippian-Pennsylvanian Lisburne Group a series of shallowing-upward depositional cycles of various magnitudes occurs. These cycles document relative sea level fluctuated during a regional transgression over the south-facing platform. Standard petrographic analysis of samples collected from the western Sadlerochit and Shublik mountains reveals transgressive vs. regressive diagenetic phases within grainstone fabrics of several cycles.
The transgressive phases are recognized in grainstone to packstone lithologies from the lower half of a depositional cycle. This phase is characterized by densely packed interpenetrated to sutured grains and crystals; grains tend to be oriented parallel to bedding. Lack of well-developed rim cements indicates compaction occurred prior to cementation by later calcite. Syntaxial overgrowths of echinoderm fragments are rare. Neomorphic textures and slow replacement by dolomite are also observed. These features are indicative of diagenesis of sediments which were subjected to a deep marine phreatic to deep burial diagenetic environment prior to significant cementation.
In contrast, regressive phases are found within grainstones at higher levels within a cycle. The regressive phase is characterized by looser packing and random grain orientation due to early cementation. Dissolution of grains, collapsed micritic envelopes, and dropped ooid nuclei are commonly observed. Fibrous marine cements are frequently overgrown by second-stage freshwater isopachous bladed rim cements, followed by late-stage calcite void-filling cement. Syntaxial overgrowths of echinoderm fragments commonly form the void-filling cement. These features indicate of diagenesis of sediments exposed to mixing-zone and meteoric diagenetic environments, respectively, during progradation of carbonate depositional environments.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990