The palynostratigraphy, age and environment of strata penetrated by the Mallik 5L-38 gas hydrate research well determined by differentiating the recycled and contemporaneous palynomorphs
Authors: White J M 
Published: February 2012

The Mallik 2L to 5L-38 gas hydrate research wells, drilled with chilled mud to reduce caving, provide 1150 m of core and cuttings, and comprise the best subsurface material available for palynostratigraphy in the Mackenzie Delta. Quantitative palynological
analysis of the kerogen fraction extracts the maximum information from the samples. It also permits the experimental approach of constructing paleoclimatic proxies by ratios of taxa in order to refine age determinations based on indicator taxa, and to correlate outside of the basin. Palynological
analysis of the Mallik 2L-38 well lacks biostratigraphic information for the interval above 670 m because of sample non-recovery during drilling (except for a core between 110 to 118 m). This study of Mallik 5L-38 from 100 to 900 m fills this biostratigraphic gap and provides 230 m of overlapping
section between the 5L- and 2L-38 so that analytic results can be compared. Core samples from coal beds between 933.65 and 1081.90 m yielded records of palynomorphs which are not recycled. The identification of the recycled palynomorphs is a challenge for this analysis. Palynomorphs are separated
into those deemed to be recycled, those deemed to be contemporaneous with sediment deposition, and unknown and undeterminable taxa. Taxa of Cretaceous age are clearly recycled, but if the source of recycling is strata not much older than the contemporaneous taxa, differentiation of recycled taxa
from those just deposited is problematic. In the preparation and analysis of Mallik 5L-38 cutting samples from 450 to 900 m, two fractions are identified in the organic matter, a +180 ?m fraction that contains a high portion of recycled taxa, and a kerogen fraction that contains recycled and
contemporaneous taxa. These two fractions are clearly differentiated by Correspondence Analysis. The differences between the +180 ?m and kerogen palynological assemblages is identified only by the analysis technique of Mallik 5L-38 and the stratigraphic extent of that differentiation, and
significance for palynostratigraphy, remains undefined in the broader context of the Beaufort-Mackenzie Basin. Correlation of the results from Mallik 5L-38 with palynostratigraphy from other Beaufort-Mackenzie wells is not straightforward because in other wells the +180 ?m fraction was likely
oxidized and released into the sample, yielding an assemblage richer in recycled taxa than the assemblages from the Mallik 5L-38 preparations. Regional tectonic and climatic events, and their ages, are interpreted by the changes in the palynomorph and lithostratigraphic record. Taxa known to be
recycled, plus the poorly preserved unknown and undeterminable taxa in the kerogen fraction above 700 m collectively record an acceleration of regional erosion, possibly representing regional uplift in the Early Miocene. Borehole erosion measured by caliper logs above 650 m also reflects
discontinuities in the percentage of recycled palynomorphs and the unknown and undeterminable palynomorphs. The Mallik 5L-38, 0 to 270 m interval is interpreted to be of late Pliocene to Pleistocene age. The interval from 270 to 340 m is dominated by the deposition of eroded material, which may
represent some late Pliocene or Pleistocene glacial event more limited than a late Wisconsinan-style continental glaciation. The sub-Iperk regional erosional unconformity is identified at 340 m, with other possible unconformities at 445 and 550 m. In the interval from 340 to 700 m there is no
evidence of an assemblage representing either the Middle Miocene thermal maximum or the subsequent period of decline of thermophilous taxa; hence, this interval is thought to be of Early Miocene age. The last appearance datums of Tsuga spp. and Baculatisporites spp. in this interval argue for a
minimum mid-Miocene age based on comparison with Nuktak C-22 and Adgo F-28 wells. Oligocene intervals are tentatively identified from 700 to 900 m, but palynological definition of the Oligocene is problematic regionally. Although last appearance datums occur in the upper Oligocene in other studies,
there is little agreement on the assemblage that marks the Oligocene. Identification of the Oligocene seems to require the recognition of intervals of more continuous occurrence of otherwise longranging taxa, Baculatisporites, Tsuga and Ericales, that occur in the Arnak Member of the Kugmallit
Formation. The interval from 700 to 850 m may represent the Late Oligocene Chattian Age, a warm climatic interval. The 850 to 900 m interval may represent the Early Oligocene Rupellian Age, a cool climatic interval. There are similarities between the pollen zonation and the biogenic/thermogenic gas
zonation. The climate proxy ratios in the overlapping section of the 2L- and 5L-38 wells between 670 and 900 m show few similar patterns. The difficulty of comparing ratios between wells may stem from under-saturated rarefaction curves for the assemblages and from low counts with wide confidence
intervals. Higher palynomorph counts are required, but may be impractical where sample volumes of cuttings are limited. It is also possible that the change from coherent, independent ratio patterns below 886 m in 2L-38 to out-of-phase patterns in 5L-38 may indicate an important stratigraphic change.
The question is raised whether, in the 2L-38 study, palynomorphs from the +180 fraction were released into the kerogen, even though by a study of processing technique, that should not have happened. The significance of laboratory processing is highlighted. An erosional unconformity about 930 m may
have resulted from sea level decline due to earliest Oligocene glaciation in Antarctica.Analysis of core samples from coal beds in Mallik 5L-38 between 933.65 and 1081.90 m yielded records of palynomorphs which can not be reasonably attributed to recycling. They confirm that Late Eocene palynomorphs
occur in this interval, and show that some palynomorphs usually attributed to older Eocene rocks range into the Late Eocene. Paleontology, sedimentology, organic carbon and geochemical analysis yield a consistent picture of an upper to lower delta plain setting with a dominance of terrestrial
organic matter and a varying marine influence. Limited previous dinoflagellate and forminiferal results are compatible with a Late Eocene age. A warm paleoclimate is indicated by the modern relatives of pollen from temperate angiosperms found in the coal, a paleoclimate consistent with an Eocene
age. Dominance of wetland assemblages by Ericale or Taxodiaceae-Cupressaceae-Taxaceae pollen may reflect the ecological consequences of water flow-through and pH variation. The coal swamps seem to be an environment where relict species can persist.