R. J. Chohamin
Short-termed, relatively small-scale fluctuations and interruptions in depositional environments can be shown to have most significant repercussions in causing complex interrelations of lithologies within the Upper Cretaceous coal bearing Fruitland Formation and associated strata of the San Juan basin of New Mexico and Colorado. Intertonguing lithologies, divergences and convergences of associated and interrelated coal beds, sudden increases or decreases in thickness, and variations in lateral extent of individually identifiable coals can be attributed to unstable or inconsistent sedimentary conditions. Minor transgressional and regressional cycling within an overall regressional stage of an epeiric sea, together with fluctuations in rates and volumes of deposition of sed ment along the shoreline and backshore areas, is evidently the cause of the complex lithologic interrelations in the Fruitland Formation in the southwestern part of the San Juan basin.
Correlation, over relatively large areas, of individual coal beds deposited under such unstable sedimentary conditions can be very difficult, even with high-density exploration drill-hole information. Economic development of multiple-seam interrelating coal complexes requires a high degree of confidence in correlation of significantly thick, persistent, individually identifiable coal seams. Proper interpretation of radiometric logs of exploration drill holes can be an indispensable aid in correlating multiple-coal seams in section view, by examination and comparison of density and natural-gamma characteristics of seams in adjacent drill holes. A "closed fencing" technique, utilizing correlated cross sections can extend correlation in three dimensions over large areas, by carrying corr lation from one cross section to another, at points of intersection, around an area until closure is made with some beginning point. Thus the degree of continuity and lateral extent of all coals within an area can be established.
AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico