J. G. Johnson

Extinction of Perched Faunas

Regression of epicontinental seas could cause extinction of benthic marine organisms simply by destruction of their habitat, but many widespread regressions have occurred without a concomitant event of extinction. Important events of extinction can be shown to correspond to relatively rapid regressions that occurred when epicontinental seas had previously attained large dimensions. Sustained transgression, such as is associated with the formation of a cratonic sequence, allows migration of marine organisms into widespread epicontinental seas, where they adapt and become stenotopic to variable degrees. Thus, these organisms compose perched faunas (new term) in the sense that their supporting environment is destined to disappear. Relatively rapid regressions that occur under these circumstances result in extinction. Relatively slow regressions, such as occur at the end of cratonic sequences, allow speciation and extinction to become diffuse and thus not recognizable as a distinct event.

Evidence for Late Devonian movement within the Roberts Mountains allochthon, Roberts Mountains, Nevada

New evidence allows better understanding of the sequence of formations and emplacement events during the Antler orogeny in the Roberts Mountains, Nevada. Lower to lower Upper Devonian strata were deformed and unconformably overlain by middle Upper Devonian siliceous shale and chert somewhere west of their present positions in the Roberts Mountains. During a younger episode of deformation, Vinini Group (Ordovician) rocks were thrust over the Devonian rocks that were still west of the present Roberts Mountains at the time of thrusting. Concurrent with this younger thrusting, the ?Webb formation accumulated at the toe of the moving allochthon in the Early Mississippian. As the basal Roberts Mountains thrust stepped eastward, it brought the whole allochthon to its present site above the ?Webb sometime in the Early Mississippian.

Method of Multiple Working Hypotheses: A chimera

The concept of multiple working hypotheses has different meanings to different scientists, but the particular essay by T. C. Chamberlin, first published in the nineteenth century, is still cited or given lip service by some as a valid scientific method. This seems to be the result of uncritical acceptance and/or the need to validate one9s own ability to be objective or fair. Chamberlin9s Method seems unrealistic and outmoded in the context of today9s science and deserves to be recognized for what it is: a chimera.

Silurian (Llandovery) downdropping of the western margin of North America

The western continental margin of North America lay west of Lower Cambrian cross-bedded quartz sandstone, west of thick sections of Cambrian-Ordovician shelly-facies shelf carbonate rocks, and west of Upper Ordovician–Lower Silurian shallow-water carbonate rocks deposited in central Nevada and southeastern California. During Early Silurian (Llandovery) time, a north-south belt, 64 km or more wide at the edge of the continental shelf, was downdropped to depths as great as 1,000 to 1,500 m, below the deepest shelly-fauna biotopes, and remained there for the rest of Silurian time. Downdropping of the Silurian continental margin may be attributed to prolonged extension in the interarc basin, some of whose island-arc remnants are now found in the Klamath Mountains of California.

Sea-Floor Spreading and Orogeny: Correlation or Anticorrelation?

A widely accepted hypothesis states that episodes of active sea-floor spreading, growth of oceanic rise systems, and eustatic rise of sea level are interrelated and occur simultaneously. Opinions vary as to the correspondence of episodes of active sea-floor spreading and associated epicontinental transgressions with orogeny. Review indicates that if. orogeny is not restrictively defined to particular short-lived events, it takes place during episodes of active sea-floor spreading when oceanic rise systems are large so that orogeny and eustatic rise of sea level correlate in time.