Pierre-Andre Bourque

Hydrothermal origin of Devonian conical mounds (kess-kess) of Hamar Lakhdad Ridge, Anti-Atlas, Morocco

Various interpretations have been proposed for the origin of peculiar cone-shaped, finely crystalline, Devonian carbonate mounds of the Hamar Lakhdad Ridge, Anti-Atlas, Morocco, ranging from shallow-water reefs to deep-water mud mounds, formed by biogenic and/or hydrodynamic processes. This study is the first integrated sedimentological and geochemical analysis of these structures. The mounds are cone shaped, steep sided, circular to subelliptical in plan view, and exhibit internal crude bedding parallel to the mound9s outer surface. They occur in a cluster of 48 mounds on top of a volcanic massif. Stable isotope analyses of first-stage isopachous nonluminescent cement have yielded marine values. In contrast, the finely crystalline carbonate that makes up the bulk of the mound and the internally sedimented mud between crusts of early marine cements have significantly lower δ 18 O values, whereas values of δ 13 C are similar to those of the early marine cement. Strontium isotope ratios also clearly distinguish the nonluminescent early marine cements from the finely crystalline material. The 87 Sr/ 86 Sr values for the former (0.707 934–0.709 392) are in or near the Devonian marine range, whereas the ratios for the latter (0.708 515–0.709 656) indicate a more radiogenic Sr source. On the basis of their lithology, architecture, isotope geochemistry, and clustered occurrence on top of a volcanic pile, we propose that the finely crystalline material that forms the mounds and the intermound beds was precipitated from hydrothermal fluids, and that accretion of the material was driven by hydrothermal venting.

Depth-related ecological zonation of a carboniferous carbonate ramp: Upper Viséan of Béchar Basin, Western Algeria

SummaryFollowing the demise of the stromatoporoid-coral reef community in Late Frasnian time, Lower Carboniferous carbonate shelf profiles possessed a ramp geometry, with major organic buildups represented by mud-rich mounds. Microfacies petrography of the exceptionally well-preserved Upper Viséan (Lower Carboniferous) carbonate ramp of the Béchar Basin, Algerian Sahara, may well contribute significantly to our understanding of the paleoecological zonation of Carboniferous non-rimmed platforms, and of the still enigmatic mounds commonly referred to as Waulsortian banks or mounds.Facies are grouped into two broad groups: (a) a mound facies group which comprises sponge wackestone-bafflestone, sponge-fenestellid bafflestone-wackestone, crinoid wackestone-packstone, and bedded flanks of intraclastic wackestone-packstone, all four facies composing the actual mud-rich mounds, and (b) a supramound facies group composed of coral-microbial framestone, crinoid packstone-grainstone, algal-foraminiferal grainstone and oolite grainstone.Calcareous algae are important bathymetric indicators and are used to delineate three bathymetric zones based on light penetration: the aphotic zone, which contains no calcareous algae; the dysphotic zone, where there is little ambient light, and which is characterized by the presence of red algae (Fasciella, Ungdarella, Stacheia, Epistacheoides) and absence of green algae; and the euphotic zone, which receives the full spectrum of sunlight, and is characterized by the occurrence of both green algae (Koninckopora, Kamaenella, Kamaena, Palaeoberesella, Calcisphaera, Anthracoporellopsis, Issinella, Exvotarisella) and red algae.Integration of algal zonation, distribution of the other biota, and recurrence of distinct assemblages, enable recognition of seven depth-related benthic assemblages. Together with the physical properties of the facies, the benthic assemblages were used to define seven bathymetric zones, from upper to lower ramp: (1) algal assemblage (upper ramp); (2) crinoid-ramose bryozoan assemblage (mid-ramp); and (3) productid brachiopod assemblage, (4) colonial rugose coral-microbial encruster assemblage, (5) crinoid-fenestellid assemblage, (6) sponge-fenestellid, and (7) sponge assemblage (lower ramp).The vertical zonation of the mud-rich mounds and associated facies differ from that reported from the classical Upper Tournaisian-Lower Viséan Waulsortian mound-bearing successions.

Paleogeography and tectono-sedimentary history at the margin of Laurentia during Silurian to earliest Devonian time: The Gaspé Belt, Québec

The Silurian–Lower Devonian sequence of the Gaspe Belt at the Laurentia margin south of the Quebec reentrant and St. Lawrence promontory was deposited during the period between the two main orogenies that created the northern Appalachians: the Late Ordovician Taconian and the Middle Devonian Acadian orogenies. Although this sequence is traditionally considered to have been deposited during a period of quiescence between both orogenies, significant tectonic activity attributed to the Salinic disturbance began during late Llandoverian (Telychian) time and persisted until the Acadian orogeny. This tectonic activity has profoundly influenced the composition and distribution of the Silurian–earliest Devonian sedimentary facies. The shelf and shelf edge history at the Laurentia margin along the Quebec reentrant–St. Lawrence promontory is summarized according to four broad phases. Phase 1 is a Llandoverian–Wenlockian regressive phase (R1) related to post-Taconian successor basin filling, that culminated with extensive carbonate platform development. Phase 2 is a late Wenlockian–Ludlovian transgressive phase (T1). Phase 3 corresponds to a later Ludlovian–Pridolian second regressive phase (R2). Phases 2 and 3 were accompanied by extensional tectonics that produced shelf faulting and block tilting, on top of which block reefs and reef complexes settled and built a reef tract all the way along the Gaspe-Temiscouata shelf. Phase 4 is an Early Devonian phase of accelerated subsidence (transgression T2) affecting the northwestern part of the segment (Quebec reentrant area), while the southeastern part (St. Lawrence promontory area) was already uplifted due to the ongoing collision between Laurentia and the western margin of Gondwana-related terranes to the south. Composition and distribution of sedimentary facies were controlled by the interaction of tectonics, sediment influx, and sea-level fluctuations. Construction of a post-Taconian–pre-Acadian palinspastic map to plot facies has proven to be basic to obtaining a realistic picture of the paleogeography of the shelf and shelf edge of the Gaspe-Temiscouata segment at the margin of the Laurentia craton during the Silurian–earliest Devonian time interval.

Waulsortian-type bioherm development and response to sea-level fluctuations, upper Visean of Bechar Basin, western Algeria

ABSTRACT An exceptionally well-developed and well-preserved upper Visean succession rich in sponge-bryozoan-crinoid mounds in the foreland Carboniferous Bechar Basin (northwestern Algerian Sahara) is described and discussed. The succession is composed of recurrent facies mosaics forming individual superposed members. An ideal member is made up of two distinct fades assemblages. The lower assemblage forms the actual mounds, and is composed of sponge bafflestone-wackestone at the base, overlain by massive sponge-fenestellid bafflestone-wackestone, and capped by massive crinoid wackestone with bedded flanks of lithoclastic wackestone. In contrast to massiveness of the mounds, the upper facies assemblage is composed of well-bedded crinoid packstone-grainstone and ooid grainstone, with local rugose coral and algal-foraminiferal banks. On the basis of benthic assemblages and nature of substrate, seven bathymetric zones are defined, and indicate that each member of the bioherm-rich formations is a shallowing-upward parasequence controlled by an asymmetrical transgressive-regressive cycle. The local curve of relative sea level for the late Visean shows 13 cycles interpreted as fourth-order eustatic sea-level changes, each cycle averaging half a million years in duration. Third-order cycles could not be identified. The platform model proposed is a distally steepened ramp, 15-20 km wide, that developed in a rapidly subsiding foreland basin. Lateral distribution of facies on the ramp depended strongly on local tectonic setting, whereas vertical development was controlled by sea-level fluctuation. The deep-water, mud-rich, sponge-bryozoan-crinoid mounds developed in 100-150 m water depths during phases of sea-level highstand, whereas deposition of shallow-water facies occurred on top of the mounds, in less than 70-80 m water depths during regressive phases. The Bechar mounds share similarities with the classical Lower Carboniferous Waulsortian mounds, but they differ in two aspects: the abundance of large sponges, which is unique, and their vertical zonation, wherein basal facies consist of sponge-dominated assemblages. The proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbon reservoirs in the Bechar Basin, because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migration, and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles.

Sponge-Constructed Stromatactis Mud Mounds, Silurian of Gaspe, Quebec

ABSTRACT Stromatactis calcilutite mounds from Upper Silurian rocks of Gaspe Peninsula, Quebec, Canada. are important constituents of a reef complex in the southern part of the peninsula. They reach thicknesses up to 115 m and had paleoslopes of about 40°. Stromatactis constitutes up to 30 percent of the rock volume. The microfacies of the calcilutite consists of agglomerates of pelletlike bodies forming a pelletoidal network surrounded by uniform mudstone. Sponge spicules are ubiquitous. By analogy with textural features of a well-documented sponge, Malumispongium hartnageli (Clark 1924) which occurs slightly above the mounds. it is suggested that the pelletoidal network of the mound calcilutite represents a former sponge network that underwent very early dingenetic change and cemen ation. Stromatactis would have originated as early marine cementation of growth cavities in the sponge network and of cavities created by decay of local uncemented sponge tissues.

Hydrocarbon Potential of the Mississippian Carbonate Platform, Bechar Basin, Algerian Sahara

The western Bechar basin contains a 1500-2000-m-thick Mississippian succession of carbonates consisting of superposed shoaling-upward sequences, whose porosity or diagenesis have never been investigated to establish the reservoir potential of these carbonates. In this paper, we present a diagenetic and porosity evolution of the Visean part of the Mississippian carbonate platform that may lead to the development of new exploration fairways in the region. A diagenetic model is proposed in which ooid and crinoid grainstones may have a reservoir potential if hydrocarbon migration occurred before the end of the Serpukhovian, prior to complete occlusion of porosity.

Marine, Burial, and Meteoric Diagenesis of Early Silurian Carbonate Ramps, Quebec Appalachians, Canada

The coeval Early Silurian platformal Sayabec and La Vieille formations covered 10,000 and 20,000 km 2 , respectively, in the Gaspe Basin in the Appalachian orogenic belt. Field studies, petrography, and carbon and oxygen stable isotopes have been used to document the diagenetic evolution of the carbonate platforms. Significant diagenetic features included marine cements, shallow burial marine to possible meteoric-phreatic cements, burial cements, pressure-solution stylolites, fractures and minor fracture-fill cements, meteoric dissolution and meteoric-vadose cements. Cementation started in the marine environment (LF1). It was minor ( 18 O = -5.3 per thousand ; delta 13 C = +3.8 per thousand ) is taken as the best value for Early Silurian marine composition. The upper part of the radiaxialoptic cement layers is composed of an initial non-luminescent, equant to prismatic spar (LF2) passing into complexly banded luminescent scalenohedral spar (LF3). These two cements constitute the bottom of cement successions in other lithofacies of both the Sayabec and La Vieille formations. They are interpreted as likely shallow marine burial cements, but a possible meteoric-phreatic origin is not ruled out. Both hypotheses rely on CL petrographic similarities with known examples, their depleted delta 18 O signature and wide range of delta 13 C values. Ferroan cements (LF4) fill over 85% of the pore space, occlusion of which was completed at burial depths of around 450 m. LF4 cements are 18 O-depleted (delta 18 O = -8.1 per thousand and -8.7 per thousand for Sayabec and La Vieille formations, respectively) when compared to LF2 (Delta nearly equal 2.3 per thousand ) and LF3 (Delta nearly equal 1.3 per thousand ) cements for both formations. The delta 13 C values for the LF2 to LF4 La Vieille cements are much higher than those of the Sayabec. This is explained by early methanogenesis related to the paleogeographic nature of the platform. Stylolitization, fracturing and minor calcite cementation, minor dolomitization and sulfatization occurred in the burial environment and postdate LF4 cements. Post-initial burial, LF5 (gravitational) and LF6 cements (delta 18 O = -10.7 per thousand and -10.1 per thousand , delta 13 C = +3.7 per thousand and +4.4 per thousand , respectively) are interpreted to be meteoric-vadose cements filling solution vugs of the La Vieille limestones in the western part of the ramp. They are related to Late Silurian uplift in the Gaspe Belt.

Synsedimentary tectonics, mud-mounds and sea-level changes on a Palaeozoic carbonate platform margin: a Devonian Montagne Noire example (France)

Abstract The Devonian sedimentary succession of the southern flank of the Montagne Noire (France) was deposited along a divergent margin. This paper is a contribution to describe and evaluate biogenic, sedimentary, geochemical and micropalaeontological features as indicators of sea-level changes and global history of the Devonian in this area. Following transgression and shallow-water environments during Early Devonian time (Lochkovian to early Emsian), biogenic mud-rich mounds with stromatactis developed during latest Emsian at the platform margin. The depth of the Devonian sea was increasing and the seafloor passed below the photic zone and the lower limit of storm wave base during the Emsian. Growth and seismic faults affected the mounds and created Neptunian cracks and crevices, quickly filled with sedimentary material (pisoids) and cements (Neptunian dykes and veins). Light and CL-microscopy, and stable isotope geochemistry show that stromatactis, cements of Neptunian dykes, veins and pisoid cortices are early marine, whereas the red finely crystalline material that forms the bulk of the mound has been cemented in the near-surface diagenetic environment, after the early marine cementation of stromatactis and Neptunian dykes and veins, by meteoric or hydrothermal fluids. The sedimentary rocks overlying the stromatactis mounds exhibit regularly condensed iron and manganese-rich layers, interrupted by the Kellwasser hypoxic horizon. These condensed deposits developed up to the Famennian in a context of carbonate gravity sedimentation and became more and more rhythmic and frequent up section. The occurrence and irregular distribution of large-scale submarine mass flows during Frasnian and Famennian times can be related to block faulting on which Lower Devonian stromatactis mounds could have been uplifted by this block faulting to form seamounts. The sea-level fluctuations detected in the southern flank of Montagne Noire are compared to the Devonian eustatic sea-level curve of Johnson et al. (1985). [Johnson, J.G., Klapper, G., Sanberg, C.A., 1985. Devonian eustatic fluctuations in Euramerica. Geol. Soc. Am. Bull. 96, 567–587.]