Abstract: A Major Unconformity and Not Much to Show for It: The Early Aptian Shuaiba Formation of Al Huwaisah Field, Oman

Volker Vahrenkamp

The top of the Early Cretaceous Shuaiba Fm in Al Huwaisah Field, north Oman, is marked by a regional unconformity. Despite a time gap of probably several million years there is no preserved pronounced karst topography, little depth of weathering, no carbon isotope evidence for exposure and only little porosity directly associated with exposure.

At Al Huwaisah field the Shuaiba Formation consists of stacked small-scale cycles of shallow marine carbonates composed of Lithocodium/Bacinella bound/wacke/packstones, midst biostromes and associated sediments. Here, the age of the Shuaiba Formation is constrained as Early Aptian by bio- and carbon-isotope stratigraphy. The overlying Nahr Umr Fm is Albian in age. Thus. exposure of the Shuaiba Formation could have lasted several million years. Little porosity in the investigated area is of depositional origin or related directly to the regional top-Shuaiba unconformity. In the micrite-rich sediments primary porosity was predominantly micron size with the exception of larger pores in grainstone units and shelter porosity in rudist biostromes. Most of this porosity was occluded by early marine and meteoric cements. Meteoric cementation also filled a first generation of secondary porosity leached from the aragonitic parts of rudist shells. A second, late leaching phase occurred probably in a meteoric confined aquifer under burial condition. This `tertiary' diagenetic porosity is constrained as post inter and intra-partical meteoric cement, post-dolomite, post-stylolite and partially post-microfracture emplacement. Subaerial exposure could have been responsible only for a first leaching event and apparently not more than minor cement generation since there is little geochemical evidence for meteoric cement precipitation during subaerial exposure.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994