A study of heavy mineral assemblages in the Unayzah Reservoir sandstones of central Saudi Arabia has
identified successive changes in provenance signature. These define four heavy mineral units that are of
regional extent and largely coincident with the four main depositional units defined by previous authors:
Unayzah C, Unayzah B, Unayzah A and the Basal Khuff Clastics. Sandstone bodies with anomalous mineral
signatures also occur, however, especially within the Unayzah B Member. These are attributed to local
supply of sand from pre-Unayzah Paleozoic sandstones exposed on the Central Arabian Arch and on intrabasinal
highs.
The stratigraphic changes in mineralogy reflect successive developments in the geography and climate of the
region and in the pattern of sand sourcing and transport. The Unayzah C sands and the majority of Unayzah
B sands were derived from the south but whereas the southerly derived Unayzah C sands appear to have
been derived from pre-existing mature sandstones, those of Unayzah B were sourced from a wider range of
rock types including crystalline basement. This contrast is interpreted as indicating that a significant hiatus
may separate the two units. The Unayzah B sands are also characterised by the common presence of apatite,
indicating that the source rocks were relatively unweathered. This observation is compatible with the glacial
origin attributed to many of the Unayzah B sediments.
A further change in provenance signature takes place at the base the newly recognised ‘un-named middle
Unayzah member’, equivalent to the base of Unayzah A of previous authors. This is associated with the onset
of red-bed sedimentation throughout the area. Unayzah A sedimentation was terminated by a fall in sea level
that led to the formation of a widespread unconformity and to the development of deeply incised valleys along
the western basin margin. In most of the study area this unconformity corresponds to the base of the Khuff
Formation, but in the east of the area, where the succession is more complete, it is believed to occur within the
Unayzah Formation, at a level equivalent to the base of the Upper Gharif Member of Oman.
By identifying lateral and vertical changes in sand provenance, heavy mineral analysis provides an important
additional tool in the stratigraphic analysis of the Permian sandstone succession of Saudi Arabia, both at the
regional scale and wand at the scale of individual reservoir sandstone successions
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