The Maravillas Formation is a distinctive chert-rich formation that was deposited in deep water on the slope and floor of the Ouachita geosyncline in West Texas. It is 40 percent black chert; 30 percent fetid calcarenite, micrite, and marlstone; 14 percent shale; 10 percent chert of other colors; 5 percent limestone-pebble conglomerate; and 1 percent dolomite. These rocks are intercalated in even beds, generally 3–12 in. thick. The formation ranges from 60 to almost 500 ft in thickness; isopach lines delineate a lens-shaped body whose axis (uncorrected palinspastically) is northeast-southwest and is parallel with tectonic strike.

In the Marathon basin the formation is divided, from bottom to top, into three members: (1) limestone and chert, (2) chert and limestone, and (3) chert and shale. Members 1 and 2 differ chiefly in that limestone predominates in member 1 and chert predominates in member 2. In many places member 2 can be divided into three units. Member 3 is predominantly shale at the southern and eastern margins of exposures, and is the Solitario Formation of Baker and the Persimmon Gap Formation of Wilson.

Calcarenite beds are composed of skeletal grains, intraclasts, pellets, and minor shale, chert, quartz, phosphate, and glauconite grains. Laminated, or graded and laminated, beds predominate. Micrite and marlstone are composed of compacted pelletal(?) and clayey pelletal(?) silt and mud. Conglomerate beds are composed of limestone and chert clasts derived from shelf equivalents of the Maravillas and older formations. Paleocurrent data and grain-size trends indicate that detritus was derived from the northwest.

Calcarenite was deposited largely by turbidity currents; conglomerate was laid by turbidity currents, debris flows, and possibly nonturbid density currents; and micrite, marl, and clay settled as a rain of sediment from weak geostrophic or nonturbid density currents. The geometry and relict texture of chert lenses and beds prove that most chert is of replacement origin, but a few beds are altered siliceous ooze. Lumpy and chalcedony-veined black chert beds in member 2 formed as submarine slide deposits.

The preservation of abundant organic material (3.7 percent organic carbon), including some petroleum, in some beds and the absence of bioturbation features indicated that sedimentation generally occurred in an euxinic environment.

The grain size in the formation is generally coarser at the bottom and finer at the top, a response to a major epeirogenic pulse that affected the shelf northwest of the geosyncline. Deposition of clay and spiculitic clay continued after the influx of carbonate had ceased.

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