Realizing Depositional Constraints and Processes - Microfacies of Carbonate Rocks - page 865

Fig. 16.19. Mixed siliciclastic-carbonate rocks. Terminology of arenites (grain-supported siliciclastic and/or carbonate rocks
consisting of more than 50% sand and/or rock fragments).
A: Conglomerate composed of arenitic matrix and rounded lithoclasts derived from the erosion of Triassic and Jurassic
carbonate rocks. The lithoclasts are partly pitted, indicating surficial dissolution. The arenite of the matrix consists of very
angular to very well rounded carbonate rock fragments and a few bioclasts. Shallow-marine environment. Late Cretaceous:
Voldöppberg, Brandenberg, Tyrol, Austria.
B: Poorly sorted calcilithic hybrid arenite composed of carbonate rock fragments (ooid grainstone, bottom; dolostone;
mudstone to wackestone, center) derived from Triassic to Late Cretaceous rocks, as evidenced by microfossils. The rock
fragments are subrounded to well-rounded; some are blackened or stained red. Most bioclasts are penecontemporaneous.
Isopachous fringes of dogtooth cement are overlain by geopetal infill of lime mudstone, which in turn is covered by blocky
cement (center of the picture). Shore zone environment. Same locality as -> A.
C: Moderately sorted bioclastic hybrid arenite consisting of carbonate rock fragments, siliciclastic grains and bioclasts.
The coarse bioclastic fraction is dominated by nerineid and actaeonellid gastropods (right). Smaller subrounded to well-
rounded, less common bioclasts are echinoderms and benthic foraminifera. The siliciclastic fraction consists of subangular to
subrounded polycrystalline and monocrystalline quartz grains and serpentine grains, indicating the erosion of crystalline
rocks. Lagoonal environment. Late Cretaceous: Nachbergalm, Brandenberg, Tyrol, Austria.
D: Transition from a bioclastic grainstone (bottom) into an arenite composed of carbonate rock fragments, siliciclastic
grains and bioclasts. The grainstone consists of rounded, coated fragments of mollusks, including rudist fragments, algae,
corals, echinoderms, benthic foraminifera and some siliciclastic sand. The grains are cemented by isopachous fringes of
dogtooth spar, overlain by blocky calcite spar. The boundary between the grainstone and the overlying arenite is accentuated
by pressure solution. Lithification of the arenite proceeded mainly by intergranular pressure solution, producing a tightly
packed texture. Shelf arenite. Late Cretaceous: Haidach, Brandenberg, Tyrol, Austria. Scale is 2 mm for all pictures. After
Sanders (1998).