Emile A. Pessagno

Paleogeography of Jurassic fragments in the Caribbean

Jurassic rocks of the Caribbean are a sampling of 100 million years of Farallon Plate history with fragments originating at diverse paleolatitudes and from varied tectonic settings. Fragments with clear paleogeographic signatures are components of the basement complexes of Duarte in Hispaniola, Bermeja in Puerto Rico and La Desirade off Guadeloupe. Paleolatitudinally sensitive radiolarian faunas document origination of Duarte as equatorial, La Desirade as higher latitude, and various Bermeja cherts as both equatorial and higher latitude. Red ribbon chert of Duarte and Bermeja of the same age, physical appearance, and lithological association are probably dismembered components of the same slab of Pacific crust. La Desirade red ribbon chert is slightly younger than the Duarte and Bermeja red ribbon chert and was deposited at higher latitude. Bermeja tuffaceous chert is also of higher latitude and probably had an arc-proximal origin. On the basis of modeled plate trajectories in the Pacific, the origin of various cherts from different paleolatitudes that end up in the same location requires different arrival times at the trench between North and South America. Based on radiolarian paleobiogeography plus indications of origin at a spreading ridge and ignoring the poorly constrained, modeled trajectories for the Late Jurassic, at least one of the higher latitude fragments may have originated in the southern hemisphere. The accumulation of multifarious chert, greenstone, and other ocean floor components was accomplished by offscraping strata transported to the subduction zone along the eastern Pacific margin and warehousing this material in an accretionary complex prior to entry of the Caribbean Plate into the gap between North and South America.

Jurassic and Early Cretaceous radiolarians in Puerto Rican ophiolite—Tectonic implications

The Mariquita Chert of the Bermeja Complex in southwestern Puerto Rico contains radiolarians ranging in age from early Tithonian to late Aptian. Local stratigraphic and structural relations indicate that Puerto Rican ultramafic rocks are as old as or older than the chert; they are thus probably Jurassic or older. These rocks are the first pre-Cretaceous rock assemblages between Cuba and La Desirade (Lesser Antilles) to be identified, and they document the presence of pre-Cretaceous oceanic crust in the northeastern Caribbean. Subduction of the Caribbean plate beneath the North American plate, ending in the Early Cretaceous, destroyed most of this old oceanic crust and created a belt of metamorphosed volcanic and ophiolitic rocks now exposed in Cuba, Hispaniola, and Puerto Rico. The maximum radiometric ages from this belt are 127 m.y. Younger rocks were superimposed on the older belt when subduction polarity reversed and the North American plate was subducted beneath the Caribbean plate from 110 to 45 m.y. ago.

Jurassic (Tithonian) radiolaria from La Désirade (Lesser Antilles): Preliminary paleontological and tectonic implications

Jurassic (upper Tithonian) Radiolaria recovered from bedded, red ribbon cherts on La Desirade, Guadeloupe, are the oldest fossils yet discovered in the Lesser Antilles. This age not only corroborates contested isotopic ages for the igneous basement of La Desirade but also demonstrates that previously reported differences in basement ages for the central and eastern end of the island are invalid. In addition, La Desirade chert yielded a higher-latitude, Northern Tethyan to Southern Boreal Realm radiolarian assemblage (indicating deposition at a minimum of 22° north or south of the Jurassic paleoequator). Because Northern Tethyan or Boreal Jurassic oceanic crust did not exist in the spreading gap between North and South America, or east to Eurasia, we conclude that La Desirade oceanic crust formed in the Pacific. The fact that no Upper Jurassic red ribbon chert has been found in the Atlantic Ocean or in the Caribbean aside from in displaced oceanic fragments in Puerto Rico and the Dominican Republic lends additional support to a Pacific origin for the oldest crustal fragment in the Lesser Antilles.

Implications of new Jurassic stratigraphic, geochronometric, and paleolatitudinal data from the western Klamath terrane (Smith River and Rogue Valley subterranes)

Combined biostratigraphic, chronostratigraphic, and geochronometric studies of the Rogue and Galice Formations (Rogue Valley subterrane, southwestern Oregon) indicate that the Oxfordian-Kimmeridgian boundary should be placed at 154 ±1.5 Ma rather than at 156 ±6 Ma as advocated in the Decade of North American Geology 1983 geologic time scale. In the Smith River subterrane (northwestern California, southwestern Oregon), well-preserved Radiolaria were recovered not only from strata overlying the Josephine ophiolite, but also from within the volcanic member of the ophiolite. U/Pb geochronometric data constrain the age of the Mirifusus first-occurrence event to after 162 ±1 Ma and to being 157 ±1.5 Ma or slightly older. Radiolarian faunal data indicate that the Josephine ophiolite originated at Central Tethyan paleolatitudes during the latest Callovian (162 Ma) and was carried northward to Northern Tethyan and Southern Boreal paleolatitudes during the Oxfordian.

Misconceptions concerning the Cretaceous/Tertiary boundary at the Brazos River, Falls County, Texas

Abstract Detailed biostratigraphic analysis (planktonic foraminifera and nannofossils) of sixteen K/T (Cretaceous/Tertiary) boundary sections in and near the Brazos River, Falls County, Texas indicates that a lithologically distinctive, coarse clastic event bed previously attributed to a meteorite impact-generated tsunami at the end of the Cretaceous was actually deposited during the early Tertiary (Danian: K/T + at least 230,000 years). A tsunami origin for this event bed is doubtful, but if a bolide-splashdown tsunami did generate the event bed, this putative meteorite impact must have occurred well into the Danian during the post-extinction, faunal recovery phase having little effect on extant foraminiferal and coccolith populations.

Late Cretaceous Reversal Sequence

A sequence of 14 reversals (that is, seven periods of reversed polarity) is observed in Maestrichtian sedimentary rocks from the Mendez Shale of Mexico and in cores from Deep Sea Drilling Project Sites 207A and 208, Lord Howe Rise. Data from a paleomagnetic investigation indicate that (1) a brief interval of frequent field reversals occurred at the close of Late Cretaceous time (Maestrichtian), (2) a short interval of reversed polarity occurred in latest Maestrichtian and earliest Danian time, and (3) the top of the so-called “Cretaceous quiet interval” occurs close to the Campanian-Maestrichtian boundary.

Significance of Mesozoic radiolarians from the pre-Nevadan rocks of the southern Klamath Mountains, California

Ribbon cherts and siliceous tuffs of the North Fork and Rattlesnake Creek terranes of the Klamath Mountains yield Mesozoic radiolarians. Rocks of the North Fork terrane were previously considered to be of Paleozoic age and those of the Rattlesnake Creek to be of Paleozoic and Triassic age, on the basis of fossiliferous limestone bodies that are now considered to be exotic blocks. In both terranes, however, red cherts that are closely associated with ophiolitic rocks contain Late Triassic radiolarians; overlying cherts and siliceous tuffs contain Early or Middle Jurassic radiolarians. The Jurassic radiolarian fauna from the North Fork terrane is similar to a fauna contained in Franciscan chert near Santa Barbara in southern California. The change in age assignment of the dominant rocks of these terranes, based on this new radiolarian data, indicates that the suture between the North Fork terrane and the Devonian rocks of the central metamorphic belt on the east probably formed during Middle or Late Jurassic time.

Upper Tithonian Vallupinae (Radiolaria) from the Taman Formation, east-central Mexico

The pantanelliid subfamily Vallupinae Pessagno and MacLeod reached its acme of development during the late Tithonian. Two new genera (Neovallupus and Supervallupus) and seven new species are described from the upper Tithonian portion of the Taman Formation, east-central Mexico; new species are Bivallupus oppositus, B. primigenus, Neovallupus dumitricai, N. modestus, Protovallupus excellens, Supervallupus haeckeli, and Vallupus laxus. In addition, the definition of the subfamily Vallupinae is emended. Many taxa belonging to the Vallupinae are short-ranging, distinctive, and cosmopolitan-occurring in both Boreal and Tethyan strata. Vallupinid taxa have proven to be useful in developing a meaningful radiolarian zonation for the upper Tithonian of North America.

FORAMINIFERAL RESPONSE TO HABITAT DISRUPTION: ARROYO COLORADO, TEXAS

In order to assess the possible effects of pollutants on foraminifera along the Texas Gulf Coast, sediment cores were taken in four areas considered to be at risk: southern Laguna Madre, Nueces Bay, the Arroyo Colorado, and Laguna Atascosa. Geochemical analyses indicate that the sediments are relatively uncontaminated. Foraminiferal analyses, however, reveal otherwise undetected inconsistencies and changes with time. First, though foraminiferal assemblages in the surface sediments of Laguna Madre, Nueces Bay, and Laguna Atascosa are typical, they are atypical in the Arroyo Colorado. In Texas estuarine environments, Ammonia often comprises a large proportion of the assemblage. But in the Arroyo Colorado, an estuarine river, the assemblage is almost all Elphidium to the exclusion of Ammonia. A second observation is that down-core in the Arroyo Colorado cores, Ammonia becomes abundant as numbers of El-phidium decrease. The assemblage at the base of the Arroyo Colorado cores is thus more typical for the Gulf Coast, while that in the surface sediments is not. This down-core change in the foraminiferal assemblage is coincident with a change in sediment grain size, both most likely due to the dredging of the Harlingen ship channel in the late 1940s. The reasons why Ammonia, an opportunistic genus that thrives in a wide range of environmental conditions, was unable to tolerate a habitat disturbance and is nearly absent in the surface sediments of the Arroyo Colorado are unclear.

Age and Geologic Significance of Radiolarian Cherts in the California Coast Ranges

A study of the Radiolaria in Coast Range cherts establishes that rhythmically bedded cherts associated with ophiolites below the base of the Great Valley Sequence are Late Jurassic (Tithonian) in age, and rhythmically bedded cherts in the Franciscan complex are of essentially the same age. The only younger pelagic sediments in the Franciscan are pelagic limestones (Laytonville and Calera types) and associated cherts of Late Cretaceous (Cenomanian-Turonian) age. In light of most plate tectonic models, the lack of pelagic sediments other than these seems enigmatic. The relation between the cherts associated with the ophiolites and the overlying Great Valley Sequence indicates a sudden cessation of pelagic sedimentation in early Tithonian time and probably reflects the rise of the Nevadan orogen. It is postulated that flysch-type turbidite sediments accumulated so rapidly during the remainder of Mesozoic time that they filled in an incipient trench system and spread far out onto the abyssal plain, thus inhibiting pelagic sedimentation.