Alan R. Niem

Mississippian pyroclastic flow and ash-fall deposits in the deep-marine Ouachita flysch basin, Oklahoma and Arkansas

Two pumiceous vitric-crystal tuffs, the Hatton Tuff Lentil and Beavers Bend tuff, occur in the deep-marine Mississippian Stanley Group. These widespread rhyodacitic tuffs range in thickness from 7 to 40 m and are separated by tens of metres of non-tuffaceous quartzose and feldspathic turbidite sandstone and shale. The tuffs consist of varying proportions of ash-sized embayed quartz crystals, plagioclase crystals (oligoclase to andesine), relict shards, volcanic dust, and altered flattened pumice fragments. Each pyroclastic unit consists of two or more tuff lithologies, including a thick lower unstratified pumiceous vitric-crystal tuff with a density-graded crystal-rich base overlain by thin-bedded pumiceous tuff and an upper massive fine-grained siliceous vitric tuff. These tuffs were probably formed by highly explosive eruptions of vesiculating acidic magma from a vent or fissure that produced incandescent avalanches of pyroclastic debris and accompanying ash clouds. The hot turbulent suspensions were rapidly quenched by sea water to form steam-inflated density slurries that flowed into the Ouachita basin. Pyroclastic flows created thick, density-graded, pumiceous vitric-crystal tuff. Numerous smaller density slurries following the main flow in rapid succession deposited the overlying bedded pumiceous tuff. Toward the end of each volcanic eruption, continuous settling of fine ash formed thick, fine-grained upper vitric tuff. Isopach maps of tuff thicknesses, an isopleth map of pumice sizes, logarithmic plots of crystal size versus distance, paleocurrent indicators, and Late Mississippian paleogeography suggest a southern volcanic source that may have been part of a magmatic arc formed at a continental margin during plate convergence between the North American plate and a southern continental(?) plate.

Detrital modes and provenance of the Paleogene Khojak Formation in Pakistan: Implications for early Himalayan orogeny and unroofing

Different tectonic settings have characteristic detrital modes and sediment-dispersal patterns. Detrital modes and sediment-dispersal patterns of the siliciclastic Khojak Formation in the Katawaz basin, Pakistan, suggest that its sand was derived from the early Himalayan orogen and longitudinally transported to the Katawaz remnant ocean, where it was deposited as a delta−submarine-fan complex. Modal analysis of the Khojak Formation suggests composition that is dominated by subangular quartz with abundant lithic fragments and minor feldspar, i.e., Qt 60 F 9 L 31 (Qt, total quartz; F, feldspar; L, lithic fragments). The predominance of quartz, sedimentary, and low-grade metamorphic lithic fragments suggests early derivation from a collision orogen; scarcity of detrital feldspar and volcanic lithic fragments precludes a magmatic arc as the main source. The decrease in monocrystalline quartz, concomitant increase in total lithic percentages, and relative abundance of low-grade metamorphic lithic fragments from the bottom to the top of the Khojak Formation reflect progressive erosional history of the early Himalaya. This history is part of a previously known major unroofing trend collectively depicted by the detrital modes of the Murree Formation, Siwalik Group, and the modern Indus fan in the Indian Ocean. These detrital modes are also related in time and space.

Discovery of the palaeo-Indus delta-fan complex

The Indus River carries sediments from the western Himalaya and deposits some of these as channel and floodplain sediments or molasse. The rest of its load forms the Indus delta at the margin of the Indian Ocean. The Indus delta passes some of its sediments to be deposited as Indus submarine fan turbidites. Thus, as elsewhere, Himalayan molasse, delta, and fan deposition are related hi tune and space. However, when we examine fluvial and marine age-range data of the older Indus deposits, in terms of this sedimentary assemblage, a major portion of its marine record is missing. The oldest known molasse along the Indus suture zone, and hi the foredeep are middle Eocene and late Palaeocene hi age, respectively. A recent synthesis of sedimentation in the northern Indian Ocean, however, shows that turbidite sedimentation started around early Miocene in the modern Indus fan, and even later in the Bengal fan. Where are the Palaeogene Indus delta and fan sediments? We suggest herein that these are preserved as the Palaeogene siliciclastic Khojak Formation hi the Katawaz Basin and eastern Makran. The modern Indus River is 2900 km long (Coumes & Kolla 1984) and has 30 000 m3 s − 1 discharge (Milliman et al. 1984) in summer (Fig. 1), twice that of the Mississippi River. The annual suspended sediment load (Wells & Coleman 1984) varies between 395 and 435 x 103 t, almost equal to that of the Mississippi. The existence of the middle Eocene to middle Miocene Indus molasse along the

Newly discovered Paleogene deltaic sequence in Katawaz basin, Pakistan, and its tectonic implications

In the Katawaz basin, Pakistan, the deltaic and turbidite facies of the Khojak Formation are the Paleogene analogue of the modern Indus River emptying into the Arabian Sea to form the Indus delta-fan system. Facies identified include upper continental slope to prodelta, distal to proximal distributary mouth bar, distributary channel, interdistributary bay, estuary, fluvial channel, natural levee, and flood plains of lower and upper delta plains. The proposed model is a fluvial-dominated, wave-modified delta that axially fed Khojak submarine-fan turbidites, exposed in the southwestern part of the basin. These sediments were eroded from the early Himalayan orogenic highlands and transported southwestward down the axis of the Katawaz remnant ocean basin.

Molasse-Delta-Flysch Continuum of the Himalayan Orogeny and Closure of the Paleogene Katawaz Remnant Ocean, Pakistan

Molasse strata on the Indo-Pakistan subcontinent and modern Indus delta-submarine fan sediments in the northern Indian Ocean were derived from the Himalaya. Earliest molasse sedimentation started around the late Paleocene, soon after formation of the early Himalayan orogenic highlands. However, turbidite sedimentation of the modern Indus fan did not begin until the early Miocene, and occurred even later on the Bengal fan in the modern Indian Ocean. Paleogene deltaic and flysch sediments that are missing from the Indian Ocean are preserved as the Khojak Formation in the Katawaz basin and the Makran area, Pakistan. This implies that the Katawaz remnant ocean was the main depocenter of early Himalayan marine sediments prior to deposition in the Indian Ocean, the Neogene depocenter. The Katawaz remnant ocean, a southwestern extension of the Neo-Tethys, was closed by the end of early Miocene time and the Paleogene delta-flysch sediments were incorporated into the Indo-Pakistan subcontinent.

Patterns of Flysch Deposition and Deep-sea Fans in the Lower Stanley Group (Mississippian), Ouachita Mountains, Oklahoma and Arkansas

ABSTRACT A southern proximal and a northern distal flysch facies are recognized in Mississippian lower Stanley strata over an area of 5,000 sq mi in the Ouachita Mountains of Oklahoma and Arkansas. Four widespread tuffs, each with distinctive lithologies, are interbedded with deep-marine turbidite sandstones and shales and serve as key units for detailed correlations of 8 sections 500 to 1,500 ft thick. The lower Stanley flysch is an ancient analog to one or more modern deep-sea fans and adjacent basin deposits. The lithologic character, sedimentary structures, bedding styles, fan-like geometry, ratio of sandstone to shale, and stratigraphic relationships of promixal and distal facies of the lower Stanley Group are similar to middle and outer margins of modern deep-sea fans and associated basin sediments off the coast of western North America. A proximal turbidite facies (probably a channeled suprafan) was deposited in the Hot Springs area of Arkansas at the same time a deep-water shale-rich facies accumulated in the southern and central Ouachitas of Oklahoma. During later Stanley time a proximal flysch facies prograded over the shale-rich facies of the southern Ouachitas of Oklahoma and represents deposition of a middle fan facies over an outer fan and basin plain facies. This proximal facies laterally changes to a distal flysch facies of apparent outer fan and basin plain deposition in the central Ouachitas of Oklahoma. The source area for lower Stanley strata was to the south-southeast, probably a northeastern continuation of the buried upper plate of the Luling thrust of Texas.

Origin of freshwater-diatom-rich pyroclastic-debris-flow deposit in a shallow-marine Tertiary forearc basin, NW Oregon

ABSTRACT An unusual freshwater-diatom-bearing pyroclastic-debris-flow deposit is present within the shallow-marine upper Eocene to Oligocene Pittsburg Bluff Formation of northwestern Oregon. The subaerially generated pyroclastic-debris flow rapidly debouched into the shallow-marine environment. The flow formed a wedge-shaped deposit, up to 3.5 m thick, that is mappable over several square kilometers. Thickness and maximum clast size decrease offshore; near the distal margins the debris flow mixed with seawater and became a high-density shelf turbidity current. Where emplaced above storm wave base, the deposit was locally reworked to form a thin lag conglomerate. Thalassinoides burrows are present in the upper 30 cm of the deposit. The rhyodacite chemical and mineral composition, regional geologic setting, and thickness and clast-size variations indicate a nearby highly explosive calcalkaline volcanic source to the northeast (i.e., Western Cascade are). This deposit documents the earliest known Cascade arc explosive event that directly affected sedimentation in the Tertiary forearc basin of western Oregon. A sanidine- and biotite-bearing ash fall tuff 3 m above the debris-flow deposit was derived from a backarc eruptive source (e.g., Oligocene John Day Formation). This tuff yielded an 40Ar/39Ar date of 29.83 Ma. The well-preserved diatom flora within the debris-flow deposit matrix extends the geologic range of some genera (e.g., Gomphonema) and widens the geographic distribution of others (e.g., Gomphopleura). Possible scenarios to explain the presence of exclusively freshwater diatoms in the debris-flow deposit in this shallow-marine section include: (1) a primary pyroclastic debris flow may have passed through a diatom-rich lake between the site of eruption and its entrance into the sea; (2) freshly erupted pyroclastics temporarily blocked a river drainage, creating an alpine lake that produced the debris flow upon dam failure; (3) primary pyroclastic debris may have been vented directly through a caldera or crater lake rich in diatomaceous sediment.