Browsing by Author "Jones, Brian"

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    Ecological controls on Devonian stromatoporoiddominated and coral-dominated reef growth in the Mackenzie Basin, Northwest Territories, Canada
    (2011) Corlett, Hilary; Jones, Brian
    The Horn Plateau Formation, composed of isolated reefs, is part of the Devonian strata that formed in the Mackenzie Basin in the Northwest Territories, Canada. The reefs stretch over a 350 m northeast-southwest trend and are dominated by tabulate and rugose corals in the northeast, near exposed Canadian Shield rocks, and stromatoporoids further out in the basin. Detailed facies analyses of each reef type shows distinct differences in their biological makeup, energy regimes, and carbonate sedimentation rates. Geochemical analyses (stable isotopes and rare-earth elements) set against established paleogeography in the Mackenzie Basin reveal that the coral-dominated and stromatoporoid-dominated reefs grew under different ecological conditions. Separations in the data imply that the coral-dominated reefs grew in waters that were relatively enriched in nutrients and the stromatoporoid-dominated reefs, further down the ramp, were in oligotrophic conditions. With no current established method to directly measure Paleozoic nutrient levels or to detect where they were sourced from, it is unclear why the coral-dominated reefs experienced higher nutrient levels. The paleogeography of the Mackenzie Basin could have affected the apparent stratification of nutrients on the carbonate ramp. Possible nutrient sources in the area are from coastal upwelling from the open ocean northwest of the ramp, or locally sourced nutrients from runoff on the adjacent exposed Canadian Shield rocks.
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    Epiphyte communities on Thalassia testudinum from Grand Cayman, British West Indies; their composition, structure, and contribution to lagoonal sediments
    (2007) Corlett, Hilary; Jones, Brian
    Thalassia testudinum, the most common seagrass found in lagoons around Grand Cayman, influences sedimentation by baffling currents, binding sediment on the seafloor, and providing substrates for a diverse epiphytic biota. About 85% of the epiphytic biota is formed of at least 3 species of coralline algae, 72 species of foraminifera, and 61 species of diatoms. The rest of the biota is formed of sponges, gastropods, ostracods, coccoliths, dinoflagellates, brown algae, and worms. The epiphytes are organized in three communities that are part of an organized tripartite community succession. The basal diatom community is overlain by the coralline algae community, which is then overlain by a community composed of a variety of taxa. The coralline algae community, which is the most extensive, typically covers ∼ 75% of the leaf's surface. Potentially, the skeletons of these epiphytes can make a significant contribution to the fine-grained sediment budget of these lagoons. Surprisingly, only a few of the epiphytes were found in the lagoonal sediment. It appears, therefore, that the epiphytes are lost through skeletal dissolution or transported out of the lagoon following storms. Irrespective of the cause, the epiphytes do not form a significant part of the lagoon sediment in Grand Cayman.
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    The influence of paleogeography in epicontinental seas: a case study based on Middle Devonian strata from the MacKenzie Basin, Northwest Territories, Canada
    (2011) Corlett, Hilary; Jones, Brian
    The history of Middle Devonian sea level changes are recorded in the Chinchaga Formation, Lonely Bay Formation, Horn Plateau Formation, and the Horn River Formations that originated in the MacKenzie Basin, which is located in the southern Northwest Territories, Canada. These strata record an almost consistent sea-level rise throughout the Middle Devonian with punctuated deepenings. Deposition began in the MacKenzie Basin in the Eifelian with restricted shallow water evaporites and dolomites of the Chinchaga Formation. Overlying the Chinchaga Formation are the open-marine carbonates belonging to the Lonely Bay Formation, which were overlain by the Horn Plateau Formation buildups. Pelagic shales of the Horn River Formation in the Late Givetian/Early Frasnian later covered these buildups. During the Early and Middle Devonian many continental basins in North America had limited interactions with open ocean waters. These restricted shallow-water basins were vulnerable to the effects of clastic shedding and evaporitic drawdown that commonly caused local lowstands. Several of these shallow basins, including the MacKenzie Basin, have been used to construct Devonian sea-level curves. Comparison of the Middle Devonian strata that formed in the MacKenzie Basin with well-established sea level curves shows that the Late-Givetian regression interpreted from some of those successions is not evident in the succession found in the MacKenzie Basin. This probably reflects the fact that the Mackenzie Basin was directly linked to the open ocean and was not influenced by the changes in sea level that affected many of the enclosed basins.