geopetal


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geopetal

[¦jē·ə¦ped·əl]
(petrology)
Pertaining to the top-to-bottom relations in rocks at the time of formation.
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Radial-concentric ooids with or without nucleus, micritic, superficial, composite,-pseudo ooids, neomorphosed and geopetal ooids are observed under microscope.
Bedding is disrupted adjacent to the tectonized contacts with basement: in Little Falls Brook and Big Brook bedding in sandstone is both steepened and tightly folded; in Goose Cove Brook geopetal structures involving partly filled brachiopod cavities in limestone demonstrate overturned, moderately steeply dipping rocks; and in Christopher MacLeod Brook bedding is both steepened and severely interrupted by small-scale faulting and fracturing.
Overturned beds, demonstrated by inverted geopetal structures in limestone immediately below the thrust, indicate eastward translation of the thrust sheet over them.
The bedded unit is poorly sorted and characterized by high abundance of tabulate and rugose corals and bryozoans, showing evidence of short-term transport and redeposition in the form of large, only slightly disarticulated fragments which are overturned and sheltering geopetal structures.
The vesicles and fractures in basalts were gradually filled up with secondary minerals, developing amygdales and veins usually showing mineral zoning distribution though, in some cases, they were not completely filled Figure 5a gives an example of the occurrence of a mixed layer of celadonite (Fe-rich mica) and non-tronite (Fe-rich smectite) and vesicles commonly lined by these mineral phases Celadonite is usually green and botroidal and has fibrous spheroidal morphology, having detectable striated birefringence under crossed nicols Nontronite is typically brown in transmitted light and does not have a discernable structure except for banding parallel to the cavity wall It occasionally exhibits geopetal features.
The V-shape of the cracks and an increase in sediment thickness over the cracks (geopetal fill) indicated subaerial exposure and the growth of algae preferentially around the mudcracks.
Thin sections show tubular root casts with cloudy cutans partly infilled by graded, geopetal calcite-silt, and micrite replaced by diffuse spherulitic calcite.
Gastropods are the dominant fossils, but there are also bivalves (some in life position), and brachiopods with geopetal infilling.
Hollabaugh 9:00 SEDIMENTATION IN VESICLES; INTERPRETATION OF GEOPETAL FABRICS IN AMYGDALOIDAL AGATES**, Jake L.