Composition and origin of stromatactis‐bearing mud‐mounds (Upper Devonian, Frasnian), southern Rocky Mountains, western Canada

Abstract

Stromatactis‐bearing mud‐mounds remain an enigmatic reef type despite being common in Palaeozoic ramp settings. Two well preserved Upper Devonian (Frasnian) mud‐mounds in the Mount Hawk Formation crop out side by side in the southern Rocky Mountains of west‐central Alberta and provide an opportunity to develop a new case study that can be compared with the other coeval examples, such as those well‐known ones in southern Belgium, as well as evaluate competing hypotheses for mud‐mound formation. The southern mud‐mound is 46·2 m thick and 38·6 m wide at the base, whilst the northern one is 53·3 m thick and 72·2 m wide at the base, and they exhibit three or four growth stages indicated by interfingering and onlapping geometries with flanking strata. The biota is diverse, but fossils only occupy 10·7% by volume, among which sponge spicules, echinoderms, ostracods, brachiopods and calcimicrobes belonging to Girvanella and Rothpletzella are the most common. Five microfacies are discriminated in the mud‐mounds: biomicrite, clotted micrite, spiculite, stromatolite and laminite, with clotted micrite comprising the largest proportion. There is no internal vertical or lateral palaeoecological zonation, and the presence of calcimicrobes and calcareous algae throughout indicates accretion entirely within the photic zone, in a deeper ramp setting seaward of a large carbonate platform to the east. Stromatactis is abundant and the cavities were mostly due to excavation by currents rather than physical collapse of spiculate siliceous sponges. Formation of lime mud involved a combination of multiple organisms, mechanisms and processes. Cyanobacteria were integral to mud‐mound frame‐building and accretion because they stabilized the surface, often permineralized to form Girvanella and provided organic matter that was decomposed by bacteria. This induced precipitation of micrite, forming early indurated rigid masses, evidenced by the presence of intraclasts, stromatactis cavities, isopachous marine cements, absence of bioturbation and rare synsedimentary brittle deformation. The same microbial components, invertebrate biota and clotted micrite occur in underlying strata, suggesting that there was a protracted period of potential mud‐mound initiation before the exact conditions arose to trigger it. The ramp setting, antecedent sea floor topography and relative sea‐level likely contributed together to control this. This study indicates that mud‐mound formation was controlled by a combination of processes, but they are essentially a microbial buildup.

Marine carbonate factories: Review and update

“…Relative changes in sea‐level do not have a large impact on the production rates of this system, because for flat‐topped M‐factory carbonate platforms the main sediment production is positioned on the upper slope (Figs 7 and 14). Zhou & Pratt (2019a,b) discussed that peloidal sediment production of Upper Devonian mud mounds takes place within the photic zone between storm‐weather and fair‐weather wave base. With sediment production extending over a wide water‐depth range (Fig.…”

Refined conodont stratigraphy at Martenberg (Rhenish Massif, Germany) as base for a formal middle/upper Frasnian substage boundary

“…13d), is typical for a microbial origin (e.g. Zhou and Pratt 2019). Therefore, we relate the short, diffuse tubular structures also to microbial biomineralization.…”

Revisiting the upper Visean mud mounds from Derbyshire (UK): the role of brachiopods in their growth

“…This is a common feature of several Palaeozoic mud mounds (cf. Pickard 1992;Keupp et al 1993;Aretz et al 2012;Cózar et al 2019a;Zhou and Pratt 2019;. Rapid accommodation creation decreases sedimentation rate, thus resulting in firmground formation: these two conditions are suitable for sea-floor colonisation by a suspension feeder benthic community consisting of bryozoans, brachiopods, and siliceous sponges.…”