Evaporite Diagenesis Models of a Continental Sabkha System: Insights From the Holocene at Dukhan, Qatar

Abstract

Inner ramp carbonate systems are characterized by micro-relief where small changes in sea level may introduce or remove marine waters. The water table is close to the surface, promoting evaporite formation in capillary zones and standing water. This results in precipitation of evaporites, with solutes derived from meteoric and marine waters. Diagenetic reactions in these ‘evaporite factories’ affect pore systems in sediments and in underlying and downdip bedrock. Here we examine the distribution and origin of evaporites in the Holocene Dukhan sabkha in Qatar, and their response to hydrochemical changes in a system driven by isolation from marine influence and input of meteoric waters. Holocene sabkha sediments 4–6 m thick (max 12 m) have accumulated in a structural low covering 107 km². Sea-level reconstruction suggests early Holocene marine flooding from the north, forming an embayment that became progressively more restricted, marked by evaporitic lakes or salinas. Prismatic gypsum, up to 7 m thick, is interpreted as a saline lake in middle portions of the sabkha. In the northeast a 12 km² salt flat is the youngest lake facies, fed by marine waters. Most of the sabkha is 1–2 meters below sea level. The water table lies 0.5 to 1.0 m below the surface (-2.5 to -3.0 m masl), fluctuating annually by c.40 cm reflecting dynamic recharge and evaporative drawdown. Gypsum is the dominant evaporite mineral (20–30%), followed by halite (8–11%), anhydrite (5%) and grain coating carbonate. Total dissolved solids in pore-waters in underlying Eocene carbonates increase with depth and exceed those in Holocene sediments, even at the height of the dry season. However, measurements of piezometric head indicate upward flow from the Eocene into overlying confining sabkha sediments, driven by shallow evaporation. This results in gypsum precipitation, depleting calcium and sulfate in shallow brines relative to source waters. Within the Holocene sediments, this depletion increases from the margins to the center of the sabkha. The sabkha is thinner towards the north, enhancing evaporative concentration of solutes and diagenetic gypsum formation. However, within the Eocene there is a gentle but significant decrease in head from north to south (c.8 cm/km), reflecting inflow of marine waters from the north. Topographic control on sedimentation and fluid flow patterns combine to control precipitation of evaporites both within restricted water bodies and as diagenetic precipitates.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015