Fault-controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west-central Alberta, Canada

The partly dolomitized Swan Hills Formation (Middle‐Upper Devonian) in the Simonette oil field of west‐central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post‐date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (T_h=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (T_peak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon‐bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more ⁸⁷Sr‐rich fluid. Fabric‐destructive and fabric‐preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume‐like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on ⁸⁷Sr/⁸⁶Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and ²⁰⁶Pb/²⁰⁴Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity.     

Evidence for deep anaerobic biodegradation associated with rapid sedimentation and burial in the Beaufort–Mackenzie basin,Canada

Formation waters of the 14 km thick late Cretaceous–Cenozoic Beaufort–Mackenzie basin were examined as part of a larger project to better understand the petroleum potential of the region, where early exploration defined petroleum reserves of 744 × 10⁹ bbls recoverable crude oil and 11.74 tcf gas. Historical water analyses (2583 samples from 250 wells drilled up to 5 km depth) were compiled and culled to remove incomplete and poor quality samples. The resultant database shows a broad range of salinity and water chemistry that has no systematic relationship with depth. Three main water types are defined, paleo seawater, and freshwaters related to a Miocene age gravity-driven flow system, and low TDS–high alkalinity waters. High alkalinity waters are isolated in overpressured fault blocks that were rapidly buried by post-Miocene Iperk shale deposition. The high alkalinities (up to 9000 mg/L) are interpreted to be related to in situ CO₂ generation through anaerobic methanogenesis in response to freshwater invasion. The dominant control on biogenic gas generation appears to be maximum burial temperature rather than the modern temperature distribution. This is consistent with the paleopasteurization model that suggests once critical burial temperatures are reached, sterilized rocks are inhibited from further biodegradation, even when temperatures subsequently drop back into the habitable zone.     

Characterization of volcanic deposits with ground-penetrating radar

 Field-based studies of surficial volcanic deposits are commonly complicated by a combination of poor exposure and rapid lateral variations controlled by unknown paleotopography. The potential of ground-penetrating radar (GPR) as an aid to volcanological studies is shown using data collected from traverses over four well-exposed, Recent volcanic deposits in western Canada. The deposits comprise a pumice airfall deposit (3–4 m thick), a basalt lava flow (3–6 m thick), a pyroclastic flow deposit (15 m thick), and an internally stratified pumice talus deposit (60 m thick). Results show that GPR is effective in delineating major stratigraphic contacts and hence can be used to map unexposed deposits. Different volcanic deposits also exhibit different radar stratigraphic character, suggesting that deposit type may be determined from radar images. In addition, large blocks within the pyroclastic deposits are detected as distinctive point diffractor patterns in the profiles, showing that the technique has potential for providing important grain-size information in coarse poorly sorted deposits. Laboratory measurements of dielectric constant (K') are reported for samples of the main rock types and are compared with values of K' for the bulk deposit as inferred from the field data. The laboratory values differ significantly from the "field" values of K'; these results suggest that the effectiveness of GPR at any site can be substantially improved by initial calibration of well-exposed locations. Received: 10 May 1996 / Accepted: 27 December 1996     

Spatial and Physical Characteristics of Rejolladas in Northern Yucatán,Mexico: Implications for Ancient Maya Agriculture and Settlement Patterns

This study evaluates the importance of rejolladas to the Ancient Maya in and around the Classic Maya center of Xuenkal, Yucatán State, Mexico. Rejolladas are collapse sinkholes with bases above the local water table. We present a spatial and physical analysis of 186 rejolladas in a 10 × 10 km area centered on Xuenkal. Basal diameters range from ∼22 to 264 m, areas range from 0.04 to 5.48 ha, and depths range from 4 to 12 m. Spatial density ranges from 0 to 8 rejolladas/km² with higher densities coinciding with known Ancient Maya settlements. Within Xuenkal, residential groups tend to be organized around and focused on the rejolladas. Some rejolladas have modified slopes that may be remnants of terraces or entry paths. High‐resolution satellite imagery analysis demonstrates that rejolladas have denser and healthier vegetation than the surrounding landscape especially in the dry season. Microclimate data demonstrate that the bottom of rejolladas has less extreme diurnal temperature ranges, lower daytime highs, higher atmospheric moisture, and significantly higher and more stable soil moisture. Based on the archaeology at Xuenkal, it appears that the Ancient Maya recognized and actively exploited these environmental microniches for intensive cultivation both locally and regionally.     

Petrology, rank and evidence for petroleum generation, Upper Triassic to Middle Jurassic coals, Central Alborz Region, Northern Iran

Upper Triassic to Middle Jurassic coals from the Alborz region of northern Iran were analyzed by reflected light-fluorescence microscopy and Rock Eval 6® pyrolysis to evaluate their regional rank variation, degree of hydrothermal alteration, and petroleum generative potential. The coal ranks in the region range from a low of 0.69%Ro_R in the Glanddeh-Rud area to a high of 1.02%Ro_R in the Gajereh area. T_max (°C) values (Rock Eval 6 pyrolysis) also increase progressively with increasing vitrinite %Ro values, however T_max is suppressed lower than would be expected for each rank ranging from 428 °C for the Glandeeh coal to 438 °C for the Gajereh coal. T_max suppression may be caused by maceral composition and soluble organics within the coal. Moderately high hydrogen indices, persistent and oily exudations from the coals during UV exposure, and traces of hydrocarbon fluid inclusions suggest that liquid petroleum was likely generated within some of the coals.     

Volcanology of the 2350 B.P. Eruption of Mount Meager Volcanic Complex, British Columbia, Canada: implications for Hazards from Eruptions in Topographically Complex Terrain

 The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total volume of 10⁹ m³ and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated to the 2350 B.P. eruption, although the post-eruption avalanche may have its origins in the over-steepened slopes created by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results from deposition in a narrow, steep-sided mountain valley containing a major river. Received: 20 January 1998 / Accepted: 29 September 1998     

Characterization of diatom-derived lipids and chlorophyll within Holocene laminites, Saanich Inlet, British Columbia, using conventional and laser scanning fluorescence microscopy

Conventional fluorescence microscopy and visible light region fluorescence microspectrometry have been combined with laser scanning fluorescence microscopy (LSFM) to examine and characterize soluble sedimentary organic matter (SOM) in Holocene diatomaceous laminites from ODP core 1034, Saanich Inlet, British Columbia, Canada. Microscopic SOM in the laminites is dominated by two components: red-fluorescing chlorophyllinite, and yellow-fluorescing, diatom-derived oils. Both are commonly preserved within the interior of siliceous diatom auxospores and appear as an abundant and intense stain on mineral matter throughout the laminites. The mineral stain is classified as matrix chlorophyllinite and matrix oil, respectively. Accessory insoluble SOM consists of dinoflagellate, prasinophyte and coccoidal alginites, and rare woody huminite, funginite, and sporinite. Visible light region fluorescence microspectrometry of chlorophyllinite reveals a predominance at λ max 670–675 nm, which is consistent with a chlorophyll a source based on comparison with reference samples of chlorophyll a and b. The consistency in λmax and spectral character of chlorophyllinite with increasing depth in Saanich Inlet correlates with a high degree of chlorophyll preservation. Its distribution in the laminites is readily imaged by LSFM using either UV (363 nm) or blue (488 nm) laser excitation and >665 nm emission. Yellow-fluorescing diatom oils have λmax ranging from 485 to 520 nm, suggesting that they may comprise up to 70% saturates. The distribution of entrapped yellow-fluorescing diatom oils as well as matrix oils is also readily imaged with LSFM using a combination of UV laser excitation (363 nm) and 510 ±40 nm emission. Laminae in the Saanich Inlet samples which are highly enriched in both diatom oils and ‘reproductive’ auxospore cells are interpreted as recording intense episodic spring diatom blooms. The observations reported here document microscopically, in situ, possibly pristine preservation of diatom oils (including possible fatty acids) at appreciable depths within Holocene organic-rich laminites. Their recognition in recent and older strata is important, for they serve as potential biomarkers for assessing long term geo-environmental changes such as water temperature. The periodic presence of anomalously high amounts of diatom-derived biological oils in laminae within the recent sediments may mark times of unusually high nutrient levels and paleoproductivity.     

Non-explosive,constructional evolution of the ice-filled caldera at Volcán Sollipulli,Chile

 A radar and gravity survey of the ice-filled caldera at Volcán Sollipulli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km³ of ice. Reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic unit) comprise a sequence which evolved from subglacial to subaerial facies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera ice. The Alpehué crater, formed by an explosive eruption between 2960 and 2780 a. BP, in the southwest part of the caldera is shown to post date formation of the caldera. Sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large-magnitude explosive eruption. Instead, lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera. The radar and gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath the ice in this region. This, coupled with the fact that most of the post-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial marginal eruptions. Sollipulli caldera has evolved from a collapse to a constructional feature with intra-caldera ice playing a major role. The post-caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time. Received: 12 June 1995 / Accepted: 7 December 1995     

Degassing during magma ascent in the Mule Creek vent (USA)

 The structures and textures of the rhyolite in the Mule Creek vent (New Mexico, USA) indicate mechanisms by which volatiles escape from silicic magma during eruption. The vent outcrop is a 300-m-high canyon wall comprising a section through the top of a feeder conduit, vent and the base of an extrusive lava dome. Field relations show that eruption began with an explosive phase and ended with lava extrusion. Analyses of glass inclusions in quartz phenocrysts from the lava indicate that the magma had a pre-eruptive dissolved water content of 2.5–3.0 wt% and, during eruption, the magma would have been water-saturated over the vertical extent of the present outcrop. However, the vesicularity of the rhyolite is substantially lower than that predicted from closed-system models of vesiculation under equilibrium conditions. At a given elevation in the vent, the volume fraction of primary vesicles in the rhyolite increases from zero close to the vent margin to values of 20–40 vol.% in the central part. In the centre the vesicularity increases upward from approximately 20 vol.% at 300 m below the canyon rim to approximately 40 vol.% at 200 m, above which it shows little increase. To account for the discrepancy between observed vesicularity and measured water content, we conclude that gas escaped during ascent, probably beginning at depths greater than exposed, by flow through the vesicular magma. Gas escape was most efficient near the vent margin, and we postulate that this is due both to the slow ascent of magma there, giving the most time for gas to escape, and to shear, favouring bubble coalescence. Such shear-related permeability in erupting magma is supported by the preserved distribution of textures and vesicularity in the rhyolite: Vesicles are flattened and overlapping near the dense margins and become progressively more isolated and less deformed toward the porous centre. Local zones have textures which suggest the coalescence of bubbles to form permeable, collapsing foams, implying the former existence of channels for gas migration. Local channelling of gas into the country rocks is suggested by the presence of sub-horizontal syn-eruptive rhyolitic tuffisite veins which depart from the vent margin and invade the adjacent country rock. In the central part of the vent, similar local channelling of gas is indicated by steep syn-eruption tuffisite veins which cut the rhyolite itself. We conclude that the suppression of explosive eruption resulted from gas separation from the ascending magma and vent structure by shear-related porous flow and channelling of gas through tuffisite veins. These mechanisms of gas loss may be responsible for the commonly observed transition from explosive to effusive behaviour during the eruption of silicic magma. Received: 24 May 1995 / Accepted: 13 March 1996