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Moodies Group
Stratigraphic range: Paleoarchean ~
Type	Geological group
Overlies	Fig Tree Formation
Lithology
Primary	Greenstone belt
Other	Sedimentary Rock
Location
Coordinates	25.8°S 31.0°E / -25.8; 31.0
Region	Barberton Greenstone Belt, Mpumalanga
Country	South Africa Eswatini
Extent	Kaapvaal Craton
Location of the Barberton Greenstone Belt

The Moodies Group is a geological formation in South Africa and Eswatini. It has the oldest well-preserved siliciclastic tidal deposits on Earth, where microbial mats flourished.

Background

Archean greenstone belts preserve the world's oldest known well-preserved supracrustal rocks. Their stratigraphy, sedimentology, and deformational history have contributed significantly to our knowledge about the crustal history of the early Earth. The volcanic and sedimentary sequence comprising the Barberton Greenstone belt, termed the Swaziland Supergroup, and is subdivided, from base to top, into the Onverwacht, Fig Tree, and Moodies Groups.

The 3.22-3.10 Ga old Moodies Group, uppermost unit of the Swaziland Supergroup in the Barberton Greenstone Belt (BGB), is the oldest exposed, well-preserved quartz-rich sedimentary sequence on earth.

Geographic Extend

The moodies group is preserved in structurally separate blocks in a heavily deformed fold-and-thrust belt. North of the Inyoka Fault, Moodies strata reach up to 3700 m in thickness

Stratigraphic Units

Moodies stratigraphy north of the Inyoka Fault The standard section of the Moodies Group is in the [[eureka Syncline]] and includes ~ 3200 m of strata. The thickest known section is in the Saddleback Syncline where exposed Moodies rocks reach 3700 m thick. The Moodies units are eight. The eight main lithologic units are recognized in the Moodies Group north of the Inyoka Fault, from base upward these are:

• A basal polymictic cobble conglomerate 0 to 100 m thick, overlying sedimentary and volcanic rocks of the Fig Tree and Onverwacht Group.

• 600-1100 m of course- and medium-grained, shallow-water, immature quartzose sandstone

• 500-1000 m of generally poorly exposed fine-grained sandstone, subordinate siltstone, and rare jaspilite and magnetic shale

• Coarse-grained quartz arenite overlain by a thin jaspilite.

• Meta basalt

• A heterogeneous sequence of interbedded coarse-grained, shallow-water sandstone, tuffaceous siltstone, and minor conglomerate in the Saddleback Syncline, and shale with jaspilite, phyllite, and sub greywacke in the Eureka Syncline;

• Cobble conglomerate in the Saddleback Syncline overlain by a fining-upward sequence, and conglomerate lenses and cross-bedded sandstone in the Eureka Syncline

• A thick sequence of siltstone and shale capped by granule-bearing sandstone, preserved only in the Eureka Syndine. Collectively, units through are termed the lower Moodies Group and units through the upper Moodies Group. The following discussion briefly describes the composition, sedimentary structures, distribution, and paleocurrent patterns of these units.

Correlations

North of the Inyoka Fault, in the Eureka Syncline/Moodies Hills Block, Dycedale Syncline, Saddleback Syncline, and Stolzburg Syncline, the preserved Moodies Group includes between 0.5 and 3.7 km of strata. Most are medium and coarse-grained sandstone but conglomerate, siltstone, rare shale, and some banded-iron formation are present in most sections. A 0-20 m thick volcanic unit, the so-called Moodies Lava, is the only regionally identifiable time datum, and precise stratigraphic correlation above or below this unit is problematic However, regional stratigraphic changes in the relative abundance of K-feldspar, chert, monocrystalline quartz, and lithic fragments allow a broad petrographic correlation north of the Inyoka Fault

See also

• Archean life in the Barberton Greenstone Belt
• Fig Tree Formation
• Onverwacht Group