Pekisko and Lake Valley Waulsortian Mounds: Origin, Growth, and Implications for Lower Carboniferous Seas
Kent C. Kirkby, Dave Hunt, and J. A. Toni Simo
Established models of Waulsortian mounds emphasize a deep, quiet water origin (below the photic zone), although more recently an allochthonous origin has also been forwarded. Detailed field and petrographic study of Waulsortian buildups from the Lake Valley Formation (New Mexico) and Pekisko. Formation (Alberta) necessitates a revised interpretation for these mound suites.
Muleshoe Mound is a particularly well-exposed Lake Valley buildup composed of five unconformity bounded stratal units. Regional changes in the geometry of mound growth phases in the Lake Valley indicate that these buildups developed in significant depositional energy, at a depth where accommodation space was a primary control on their architecture (e.g. <50 m). In the mound cores, much of the micrite originated as microbial precipitates within an organic host (?algal). Microbial process constructed significant depositional relief and rigid interconnected micrite masses. Growth forms of the micrite masses vary laterally and vertically in a series of discrete fades tracts. Similar fades occur in the Waulsortian of Alberta, Montana, and Kentucky. The observed variation of microbial gr wth forms correlates with independent evidence of depositional energy such as winnowing, abrasion, intraclasts, current alignment of skeletal components, and bryozoan taphonomy.
Correlative intermound and encasing basin strata are dominated by anaerobic to dysaerobic strata alternating with subordinate oxygenated intervals. The common and widespread Lower Carboniferous association of microbial mounds, carbonate ramps, and oxygen-stressed basin strata suggests a tendency towards high nutrient, low oxygen conditions. Waulsortian mound studies support the contention that Lower Carboniferous seas provide an alternate paleocean model, quite distinct from present conditions.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California