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Nested Cyclicity in Mudrock Successions: Bone Spring Formation, Delaware Basin, West Texas


Bone Spring (Leonardian) mudrock successions in the Delaware Basin vary cyclically in composition between silica-enriched and carbonate-enriched facies. These stratigraphic cycles occur at multiple vertical scales that may record influences of tectonics, climate, and sea-level change at the larger scale. At the smallest scale cycles may represent relatively instantaneous depositional events, or background sedimentation punctuated by influx by sediment of alternate composition. Core-calibrated geophysical well logs are useful for mapping meter-scale intervals over regionally, whereas high-frequency cycles are likely to only be evident in core. In a 300-ft (91 m) core from Reeves Co. TX, Bone Spring cycles composed of siliceous-calcareous mudrock couplets range in average thickness from 10.6 ft (3.23 m) in siliciclastic-rich lowstand successions, to 3.5 ft (1.07 m) in carbonate-rich highstand successions, based on X-ray fluorescence (XRF)-based mineralogical variations defined from measurements at average spacings of 0.8 ft (0.24 m). XRF analyses of laminated mudrock facies measured at 0.1-mm intervals reveal mm-scale sedimentary cycles comprising couplets of carbonate-enriched- and silicate-enriched facies. These higher-frequency mm-scale cycles probably represent the thinnest depositional features within which individual particles are organized. The vertical resolution acquired by XRF-scanning is practically unobtainable by other methods. These measurements and interpretations provide guidance for petrographic and SEM analyses that address the processes that are responsible for development of carbonate-silicate couplets. These processes may include 1) textural sorting during a single depositional episode; or 2) episodic interruption of background sedimentation by deposition of lamina that are lithologically dissimilar to that deposited by background processes. In addition to defining primary depositional components of mudrock depositional systems, high resolution XRF analysis illustrates the challenges in characterizing mudrock successions with widely spaced measurements where vertical facies variability may be greater than conventional sample spacing.