TOOMEY, NIALL D.¹, EVAN K. FRANSEEN², and ROBERT H. GOLDSTEIN^1
1University of Kansas, Lawrence, KS
²Kansas Geological Survey, Lawrence, KS

Abstract: Comparison of Coastal Versus Distal Onlap in Upper Miocene Carbonates of Southeastern Spain

Upper Miocene (Tortonian-Messinian) temperate-to-tropical carbonates in the Cerro de Ricardillo area, southeastern Spain, preserve 190 m of paleotopography on basement substrate. Onlap of time-equivalent shallow-water coastal and deeper water distal strata have distinct and predictable facies patterns.

Distal onlap is characterized by eight 2-4m cycles of coarse-grained red algal-bryozoan packstone fining upward to crossbedded red algal-mollusk-bryozoan packstone. Paleotopography indicates water depth of at least 190 m. These cycles exhibit no lateral facies changes approaching the point of onlap.

Coastal onlap occurs on two sides of an isolated basement high. One side consists of four onlapping cycles typically composed of packstone dominated by whole unabraded mollusks, fining up to packstone, commonly trough crossbedded and dominated by fragmented abraded mollusks and bryozoans. Cycles thin, increase in number and lose crossbedding as deposits are traced slightly downslope. Onlapping strata on the other side consist of similar, but less well-developed cycles. Two lower cycles grade updip into serpulid boundstone mound facies (subtidal; 10 m maximum water depth) that onlap volcanic basement. Two upper cycles onlap and drape the serpulid mounds and volcanic basement. Although shoreline sediments are not preserved, onlap in these shallow-water areas likely indicates base-level rise.

Distal onlap is most easily identified by the lack of lateral facies changes approaching the point of onlap, whereas coastal onlap is identified by predictable lateral facies changes. These characteristics should be useful for distinguishing between the two modes of onlap in carbonate systems elsewhere.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas