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Permian (Guadalupian) Radiolarian Faunal Variations and Geochemical Variations in the Lamar Limestone, Delaware Basin, West Texas: Implications to Radiolarian Paleoecology, Orbital Forced Rhythmites, and Regional Paleoenvironmental Changes
AdvisorNoble, Paula J.
Geological Sciences and Engineering
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Geochemical proxies integrated with well-preserved radiolarian data from the Lamar Limestone and lower part of the Reef Trail member of the Bell Canyon Formation provide insights into paleoceanographically and climatically driven controls on radiolarian distribution in the northern part of the Delaware Basin of west Texas, during late Guadalupian time. Data from a section of Lamar Limestone (ca. 8.8 m; around 500 kyr interval), indicate that major variations between sphaerellarian-dominated and <italic>Follicucullus</italic>-dominated faunas appear to be controlled by fluctuations in nutrient supply and salinity. Geochemical parameters of bulk carbonate carbon and oxygen isotopes (&delta<super>13</super>C<sub>carb</sub> and &delta<super>18</super>O<sub>carb</sub>), organic carbon isotope (&delta<super>13</super>C<sub>org</sub>), and total organic carbon (TOC) were analyzed and show variations on two scales. On a fine scale, sphaerellarian-dominated beds are associated with a relatively siltier lithology, higher radiolarian richness, and higher TOC, and are interpreted as the result of a rise in productivity stimulated from increased terrestrial input. On a broader scale, &delta<super>18</super>O<sub>carb</sub> variations in limestone samples are interpreted as a proxy for paleosalinity in response to changes in basin circulation. Mg/Ca ratios from the carbonate fraction were observed to co-vary with &delta<super>18</super>O<sub>carb</sub> and additionally may be a useful paleosalinity proxy in this basin. The limestone beds in the middle part of the section have lighter &delta<super>18</super>O<sub>carb</sub> values compared to limestone beds above and below, and likely represent the interval when the Delaware Basin was less restricted and had relatively normal marine salinity. Impacts of diagenesis are mild but are likely the cause for systematically light &delta<super>18</super>O<sub>carb</sub> values in silty limestone beds in the middle interval. The Mg/Ca ratios and &delta<super>18</super>O<sub>carb</sub> values in the limestone samples co-increase from the middle to the top of the section, implying a gradually enhanced evaporitic environment coupled with increased salinity towards the top section. Collectively, these data paint a picture of intermittent restriction in the Delaware Basin, prior to change to a dominantly evaporative regime in the late Permian, and on a finer scale, show dramatic and frequent ecological fluctuations that appear to be driven by fluxes in terrestrial input and, to some extent, paleoproductivity.The Lamar Limestone shows rhythmicity that mimics orbital cycles, further suggesting a climatic control for the Lamar faunal fluctuations. Spectral analyses on the radiolarian relative abundance and geochemical data using multiple techniques reveal a common period of ca. 116 m/cycle for all data, and another period of ca. 480-550 cm/cycle for three of the geochemical proxies. Average spectral misfit calculation applied on the gray scale data implies that average post-compaction rock accumulation rates of around 1.5 cm/kyr provide the best fit of the data to orbital frequencies, This accumulation rate is corroborated by statistical tests and estimations of accumulation from stratigraphic thickness of the Capitanian strata in the Delaware Basin. The upper Lamar Limestone appears to record the eccentricity (ca. 100 kyr), precession (16-21 kyr), and possibly sub-precession cycles (5-11 kyr), with apparently depressed obliquity influences. Given the paleogeographic and paleoclimatic conditions over the western equatorial Pangea during Permian, the precession-related monsoonal circulation could have transferred the orbital signals into these rhythmites through changes in precipitation and run-off, which ultimately affected the radiolarian fluctuations. Aside from environmental interpretations, the &delta<super>13</super>C<sub>carb</sub> values show a distinctive decreasing trend across the boundary between the Lamar Limestone and Reef Trail members that serve as a chemostratigraphic correlation tool, and are consistent with the a lowstand event during the deposition of the base of the Reef Trail. Two sections preserving the Lamar-Reef Trail boundary along the basin margin show a decline in over 2.5 per mil (VPDB) in the &delta<super>13</super>C<sub>carb</sub> relative to a fairly stable heavier baseline &delta<super>13</super>C<sub>carb</sub> value in the underlying Lamar Limestone. In the more basinal sections, the Lamar Reef Trail boundary is more subtle and has been speculated to occur at the base of wavy beds. Close sampling across a wavy bedded interval in two sections also show this distinctive &delta<super>13</super>C<sub>carb</sub> shift providing geochemical corroboration for the placement of base of the Reef Trail Member at the base of the wavy beds. Accompanying &delta<super>18</super>O<sub>carb</sub> values vary in a range generally overlapping the seawater composition, and only a slight negative offset is found in the base Reef Trail. On the other hand, the basal Reef Trail samples show a covariation of &delta<super>13</super>C<sub>carb</sub> and &delta<super>18</super>O<sub>carb</sub>, while Lamar samples do not. These relationships may indicate a reservoir isotope composition change and probably increased meteoric fluid influences in shallow water associated with a low-stand system track during the basal Reef Trail equivalent time. Direct diagenetic impacts on the examined samples appear to be negligible.