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Abstract

The Graneros Shale at Pueblo is typical of the formation throughout the southern Colorado Front Range. It is comprised of three members: The lower shale member, middle Thatcher Limestone Member, and upper shale member. The Graneros is generally represented by dark gray, slightly to moderately silty, well-laminated, non-calcareous clay shale containing scattered low diversity benthic faunas. Many parts of the Graneros lack faunas or bioturbation. These features, and relatively high levels of organic carbon (1 - 4.2 %/wt.) suggest a predominance of quiet water, oxygen-depleted (dysaerobic), benthic environments in proximal to medial offshore settings below wave base. The Graneros Shale preserves abundant event marker beds, enhancing precise regional correlation; bentonites are especially abundant and reflect the waning stages of the largest Cretaceous (Lake Albian) volcanic episode. High-resolution stratigraphic analysis of the Graneros Shale demonstrates dynamic environmental and paleoceanographic changes during its deposition, as follows: Well-circulated, oxygenated benthic substrates within reach of storm wave base (Facies 1; base of lower shale member) grade upward into; (2) quiet water, oxygen-depleted benthic environments below wave base (lower source rock interval); to (3) moderately well-circulated and partially oxygenated substrates in quiet water settings, preserving little organic carbon at the top of the lower shale member; to (4) a rapid incursion of Subtropical, oxygenated, normal marine waters with accelerated eustatic rise; calcareous shales and pelletoid limestone characterize this interval and contain a moderately to highly diverse warm water benthic and pelagic biota (the Thatcher Limestone Member); to (5) quiet water, oxygen-depleted benthic environments preserving abundant organic material in medial offshore settings (upper shale member); and at Pueblo (6), a recirculation event in the seaway associated with incursion of warm southern waters, normalization of oxygen and salinity, and development of diverse benthic faunas. This event marks the first phase of a long pelagic carbonate-producing episode that characterizes the overlying Greenhorn Formation. Fourth-order eustatic fluctuations are thought to be the major controlling factor in major facies changes during deposition.

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