The Miraleste Tuff Bed is a lapilli tuff unit in a tongue of diatomaceous shale in the Monterey Shale. Restudy suggests that the Miraleste is a complex of tabular bodies ∼1 mm to 3 m thick. The bodies occur at almost the same stratigraphic horizon through ∼10 km of exposure; but in detail they are gently to sharply discordant. The tuff consists of pumice lapilli typically 1 to 2 cm across in a matrix of tuffaceous and diatomaceous debris. The cross-cutting relations, as well as brecciation of the diatomite and incorporation of diatomite xenoliths, indicate that the tuff is intrusive. The intimacy of intrusion and the mixed components of the matrix suggest gas transport of the intrusive debris.
Characteristic, regular laminae in the diatomite are progressively disrupted toward the highest stratigraphic levels of intrusion, but they resume with typical regularity above the intrusions. The suggestion is that the host diatomite lay just beneath the sea floor at the time of intrusion. Consequently, low effective stresses in the diatomite encouraged the development of horizontal conduits. Fluidization studies show that gas velocities sufficient to transport poorly sorted debris through vertical conduits may be insufficient for transport through horizontal conduits. The preferential emplacement of grossly concordant sills in the diatomite may thus be explained.