Abstract

In the southern Ahnet Basin of Algeria a unique Middle Devonian underwater scenery is revealed, which consists of 191 mud mounds, 15 mud ridges, and 2 mud "atolls" distributed over nine discrete areas of a shallow-pelagic carbonate shelf. All the buildups are totally exhumed, thus perfectly exhibiting their original morphologies, slope angles, elevations, and the pattern of the onlapping strata as well as their relationship to the surrounding off-buildup sequences. Types of buildups include (1) small mud mounds (1-5 m high, 10-20 m diameter), (2) large mud mounds (up to 40 m high, up to 200 m diameter), (3) lens-shaped mounds(1-30 m thick, 10-15 degrees slopes), (4) mud ridges (up to 100 m high, 150-8500 m long) and (5) semicircular mud "atolls" (2500-2800 m long, 20-30 m high). The buildups were constructed during a short time span in the early Givetian (part of the Lower varcus zone), probably not exceeding a few hundred thousand years. Sedimentation rates in coeval off-buildup areas were at least 3-10 times lower. The lithology of the buildups is a massive boundstone with a high proportion of irregular, spar-filled cavities (stromatactis) but only very scattered potential frame builders. The carbonate mud is considered to be an in situ, cyanobacterial precipitate, although evidence of such is poor. Organic components of the buildups are not significantly different from those of the off-buildup lithologies, and include numerous crinoid ossicles, some tabulate and solitary rugose corals, brachiopods, mollusc debris, trilobites, and very rare sponge spicules and bryozoans. The total absence of stromatoporoids, large tabulate and colonial rugose corals, green algae, and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone (100-200 m). They are onlapped by a discontinuous sequence, a few meters thick, of skeletal packstones and grainstones, and locally, orthoceratid floatstones that were deposited later in the Givetian. A widespread rise of sea level in the early Frasnian buried both buildups and adjacent areas under shales and siltstones. Postdepositional alteration of the mud buildups includes: (1) formation of stromatactis cavities, probably by dewatering and internal erosion, (2) cementation of stromatactis cavities and interparticle pore spaces by early high-Mg calcite and late, blocky low-Mg calcite, (3) neomorphic transformation of Mg-calcitic mud and aragonitic skeletal debris, (4) occasional formation of neptunian dikes, (5) slight erosion of buildup surfaces, (6) compaction and slumping of onlapping capping beds, and (7) local dolomitization. The formation and growth of the mud build-ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds. By continuous vertical and lateral growth, closely spaced mounds amalgamated into mud ridges and mud "atolls". Regular NW-SE and N-S patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late Precambrian tectonic lineaments, which reappear in the Hercynian orogeny. The isotopic signature of matrix and early cements of mound rocks, however, does not support the idea of fluid migration and hydrothermal CO 2 supply during buildup growth.

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