STRATIGRAPHY AND ERUPTION DYNAMICS OF CRATER ELEGANTE, PINACATE VOLCANIC FIELD, SONORA M­­EXICO

Crater Elegante is a young (~42-32 ka) maar volcano in the Pinacate Volcanic Field (Mexico), formed during phreatomagmatic eruptions as rising magma interacted with groundwater to create the crater itself and the encompassing pyroclastic deposits. Due to the arid environment, the volcano is well-preserved, and erosion has been limited largely to deep radial gullies that expose a near-complete stratigraphic sequence of the eruption. Because of these characteristics, Crater Elegante provides an excellent opportunity to reconstruct the eruptive history of a nearly axisymmetric maar and interpret outcrop deposits in terms of eruption dynamics.

We completed four field campaigns consisting of stratigraphic documentation of more than 30 outcrops around the crater. We divide the volcano into four sectors, each with distinct characteristic sequences. The Western sequence begins with a thick scoria fall unit which was partially eroded as the pyroclastic surge and flow traversed over it. The North-Northeast sector consists of three massive ash units that can be traced continuously to the distal edge of the deposit > 600 m from the crater rim, and by regions of blocks and bombs as far as >300 m from the crater rim. In the East sector, three main surge units are separated from one another by thin layers of lithic lapilli fall, providing evidence of multiple pulses of surge emplacement followed by fall deposits possibly generated during the opening of new vents. The South sector is dominated by numerous cross-bedded ash layers and an overlying thick layer of dark lapilli and blocks separated from the surge units by a sharp erosional surface. This reworked dark unit may indicate a wet episode significant enough to rapidly mobilize the dark deposit from higher elevations nearer the vent. Additionally, a thin, fine-grained pyroclastic unit can be found ~10 km SE of Crater Elegante, though it is yet to be determined if it is a primary or reworked surge deposit. If it is primary, it supports the hypothesis that pyroclastic surges may have greater runout distances than the currently accepted maximum of 6 km. Overall, these sequences support the interpretation that the eruptive center migrated repeatedly during the eruption, creating a variety of eruptive sequences around Crater Elegante despite its axisymmetry.