Page 141 - Carbonate Sedimentology and Sequence Stratigraphy

132 WOLFGANG SCHLAGER

mentation alternates between basin center and basin mar- They may occur in all systems-tracts but with different prob-
gin. The basin is sediment-starved when the carbonate plat- ability (Fig 7.32). Thick evaporites may form lowstand de-
form aggrades and progrades most rapidly; on the other posits when silled basins become nearly isolated and ther-
hand, thick siliciclastic units in the basin may have no equiv- fore hypersaline during low sea-level positions. In trans-
alent on the basin margin. Reciprocal sedimentation is a log- gressive systems tracts, significant evaporite bodies may
ical consequence of the principles of highstand shedding of form when the lagoonal depression of a former highstand
carbonates and lowstand shedding of siliciclastics. is being flooded. Highstand systems tracts typically con-
In the platform interior parts of highstand and transgres- tain evaporites in the sabkhas at the landward end of ma-
sive systems tracts, siliciclastics are somewhat more com- rine deposition. Finally, gypsum/anhydrite deposits may
mon than at the platform margin. In the Permian of the occupy all depositional environments if the water is hyper-
Guadalupes, highstand platform cycles are “cleaning up- saline but sea level still high enough to flood the former car-
ward”, i.e. they are quartz-rich at the base and become clean bonate shelf. According to the systems tract definition in
carbonate at the top (Fig. 7.31). The occurrence of evaporites Fig. 7.3, this would be a highstand tract largely composed of
in carbonate sequences has been reviewed by Sarg (2001). evaporite deposits.

Fig. 7.31.— Distribution of siliciclastics (mainly quartz sand and silt) in the Permian of the Guadalupe Mountains. Major siliciclastic
accumulations in the deep basins during lowstands, when the platform is mostly exposed; minor tongues of siliciclastics accumulate
between platform carbonates of the transgressive and highstand tracts (“cleaning-upward cycles”). After Sarg et al. (1999), modified.

shelfward basinward

SB
SMW
HST
SB SB

sabkha halite LST
subaqueous siliciclastic -rich
gypsum/anhdydrite wadi deposits SB

Fig. 7.32.— Overview of evaporite occurrences in carbonate sequences. Evaporite bodies (shaded) may occur in all major facies
belts and in all systems tracts. However, they are most common in lowstand and transgressive systems tracts. During basin-wide
hypersalinity, continuous anydrite formations may replace carbonates in all settings, including the shelf margin. After Sarg (2001),
modified.