Intracaldera volcanism and sedimentation- Creede Caldera, Colorado

Abstract

Within the Creede caldera, Colorado, many of the answerg to its postcaldera volcanic and sedimentary history lie within the sequence of tuffaceous clastic sedimentary rocks and tuffs known as the Creede Formation. The Creede Formation and its interbedded ash deposits were sampled by research coreholes Creede 1 and 2, drilled during the fall of 1991. In an earlier study of the Creede Formation, based on surface outcrops and shallow mining company coreholes, Heiken and Krier (1987) concluded that the process of caldera structural resurgence was rapid and that a caldera lake had developed in an annulus ("moat") located between the resurgent dome and caldera wall. So far we have a picture of intracaldera activity consisting of intermittent hydrovolcanic eruptions within a caldera lake for the lower third of the Creede Formation, and both magmatic and hydrovolcanic ash eruptions throughout the top two-thirds. Most of the ash deposits interbedded with the moat sedimentary rocks are extremely fine-grained. Ash fallout into the moat lake and unconsolidated ash eroded from caldera walls and the slopes of the resurgent dome were deposited over stream delta distributaries within relatively shallow water in the northwestern moat, and in deeper waters of the northern moat, where the caldera was intersected by a graben. Interbedded with ash beds and tuffaceous siltstones are coarse-grained turbidites from adjacent steep slopes and travertine from fissure ridges adjacent t o the moat. Sedimentation rates and provenance for clastic sediments are linked to the frequent volcanic activity in and near the caldera; nearly all of the Creede Formation sedimentary rocks are tuffaceous. Purpose and Conclusions:The processes occurring within calderas after their formation may affect their thermal state. Structural deformation of larger calderas accompanies resurgence of the underlying magma body, which raises the Version ofJune 14,1995 1 isotherms within and below caldera, and fractures and faults the overlying caldera-fill deposits, enhancing fluid circulation. Postcaldera eruptions of gas-poor lavas and ash provide yet more heat to the system, greatly aff+ect the type of lacustrine sedimentation in the crater, and provide a window to the petrology of the cooling magma body. Within the Creede caldera, Colorado, many of the answers to its postcaldera volcanic and structural history lie within the moat deposits consisting of lacustrine clastic sedimentary rocks and tuffs known as the Creede Formation. The entire Creede sequence has now been penetrated by coreholes Creede 1 and 2 (CCM-1 and CCM-2; Fig. 1).Phil Bethke and Jeff Hulen, was to evaluate the history of intracalderavolcanic activity and sedimentation and to determine if the clues concerning post-caldera volcanism and thermal history are preserved in any way in the intracaldera lacustrine moat deposits.occurring within a few thousand years after caldera formation. Deposition of the Creede Formation was limited to a ring Urnoat" lake. Intracaldera eruptions that produced as...

Stratigraphy, correlation, depositional setting, and geophysical characteristics of the Oligocene Snowshoe Mountain Tuff and Creede Formation in two cored boreholes

“…The common observation of pumice lapilli along laminae suggests that the lapilli sank later after becoming waterlogged. The overall paucity of pumice lapilli may also reflect the importance of hydrovolcanic processes (Heiken et al, 1994).…”