Kelvin S. Rodolfo

Paleomagnetism of Palawan, Philippines

Abstract Paleomagnetic studies have been carried out on Palawan and on the island of Busuanga to the north. Results from the Cretaceous Espina Basalts of the Calatuigas Ophiolite in the South Palawan Block (SPB) pass a fold test, yield normal and reversed directions with a magnetic intensity and AF demagnetization characteristics consistent with a primary TRM. The mean direction is 293.9° and an inclination of 5.8°, with a k of 37.7 and an α 95 of 12.6°. This suggests that these ophiolites have moved northward and rotated counterclockwise by 66°±+13° with respect to the geocentric axial dipole (GAD) field. It also suggests that they were obducted from the south. Paleomagnetic directions from the Jurassic Busuanga Cherts and the Cretaceous Guinlo Formation from the island of Busuanga in the North Palawan Block (NPB) and from the Guinlo on the main island of Palawan are similar, fail regional fold tests, and have AF demagnetization characteristics consistent with secondary magnetization. Their inclinations are statistically indistinguishable at a 95% significance level, but variation in declination suggests differential local rotation about a vertical axis. The paleolatitude is comparable to that of regions of pervasive Cretaceous remagnetization in the South China borderland and may reflect similar remagnetization, consistent with the NPB’s proposed South China origin.

Origin and early evolution of lahar channel at Mabinit, Mayon Volcano, Philippines

Mabinit Channel, 5 km long, 15 to 70 m wide, and 2 to 22 m deep, was formed by lahars on the southeastern slope of Mayon Volcano during its 1984 eruption. In 1985, the channel was drastically modified by a lahar triggered by a typhoon. Sediment-budget calculations from surveys conducted in 1985 and 1986 corroborate inferences from other volcanoes that lahars can grow significantly in volume by eroding their channels. The 1984 eruption produced a deep summit ravine and, at its base, a fan of avalanche and pyroclastic-flow deposits 2 km2 in area and 40 x 106 m3 in volume. Lahars initiated in the ravine by heavy rains from eruption updrafts carved out 5-km-long Mabinit Channel along the west margin of the pyroclastic-avalanche field, a position that could not have been predicted from pre-eruption topography. By eroding the channel, the lahars generated more than half of their own solid contents. Below 240 m, where the volcano slope decreases from 5° to 3°, the first eruption debris flows were unconfined and spread out over a 512,000-m2 area, depositing a layer locally more than 4 m thick with a volume of 1.25 x 106 m3. The thin margins of these unconfined flows were non-erosional; however, as they stopped, their thick central portions continued flowing, extending the new channel downslope through the depositional field. Subsequent eruption lahars modified the channel and overflowed at its bends. A moderately intense typhoon in October 1985 triggered a single 9-hr lahar event that widened the channel by an average of 25 m and caused as much as 66 m of lateral bank erosion. Debris flows overtopping the channel at four sites coalesced to cover a 2-km-long, 200,000-m2 area with bouldery sediment. In wider channel stretches, debris-flow margins left prominent paired levees, while their central portions continued moving down-channel as rigid plugs. The lahar plugged the lower 0.5-km stretch of the channel and replaced it by eroding a new channel of comparable size. This demonstrated how easily lahars can change the courses of channels on the upwardly convex debris apron of a stratovolcano, with serious consequences for farmland and communities.

Sedimentologic and dynamic differences between hot and cold laharic debris flows of Mayon Volcano, Philippines

Numerous hot lahars produced during and immediately after the 1984 eruption of Mayon Volcano, along with frequent cold lahars of the succeeding wet-monsoon seasons, have left widespread debris-flow deposits. Fresh deposits of hot debris flows have thin crusts that are remarkably resistant to erosion and to vegetal growth; these crusts contain clasts with uniform thermoremanent magnetization directions, rare soft-sediment deformation structures, and gas-escape tubes. Deposits of unchannelized hot and cold debris flows both have sheetlike or lobate geometry, non-erosional bases, poor sorting, relatively minor silt and only traces of clay-sized ash, either matrix or clast support, and common out-sized clasts at their tops. It is difficult to distinguish between the deposits after burial, but they have subtle, significant sedimentologic differences. The inversely graded bases of hot debris-flow deposits are thick, suggesting a lack of cohesion and considerable dispersive pressure; they have long clast axes aligned parallel to bedding due to strongly sheared laminar flow, and generally they are succeeded by gradual normal grading from waning-flow conditions. In cold debris-flow beds, only the basal few centimeters may be inversely graded, the bulk being commonly ungraded, indicating the active role of cohesive forces. In the hot flows, vaporization of water by heat from large clasts may have facilitated mobility by decreasing internal friction. Indirect velocity calculations indicate that they moved faster and traveled farther than did cold flows. Proximal hot debris flows were all supercritical, with Froude numbers decreasing enroute. The most distal hot debris flows and all of the cold ones were subcritical. Cold debris flows show no correlation between Froude number and distance of transport.

Sedimentary facies and tectonic implications of Lower Mesozoic alluvial-fan conglomerates of the Newark Basin, northeastern United States

Abstract Upper Triassic to Lower Jurassic conglomeratic sequences of the Passaic and succeeding formations along the northwest-bounding fault of the Newark Basin in eastern Pennsylvania, New Jersey and New York, have six relatively distinct lithofacies that record continental basin filling under arid to semi-arid conditions. Matrix-supported conglomerates represent debris flows: clast-supported conglomerates are hyperconcentrated steamflood deposits. Coarse pebbly sandstones represent a braided-stream channel facies, and medium-to-fine sandstones are interpreted as a sheetflood facies, deposits normally succeeded by thin mudstones of a waning-flood facies. Associated with such mudstones at Milford, New Jersey is a sixth, pedogenic facies: caliches that document pauses in deposition. Areal lithologic variations and analyses of vertical changes, including Markov analyses of facies transitions, document intermittent, cyclical debris flows and streamfloods on alluvial fans prograding into the basin. The intermittency is best accounted for by episodic, localized tectonic rejuvenation of fan provenance at the northwest bounding fault of the Newark Basin during the late Triassic to early Jurassic, the final phase of basin filling. Penecontemporaneity of tectonism and fan deposition is corroborated by deformation of some conglomerate clasts which transmitted tectonic stresses to each other before their enclosing matrices were completely lithified. Some pebbles and cobbles are shattered at points of contact, and clasts of relatively soft lithologies became pocked while grinding against harder fragments during post-depositional tectonic episodes.

Geochemical evidence for sundering of the West Mariana arc in miocene ash from the Parece Vela Basin

Abstract Glass and mineral fragments from discrete volcanic ash layers were sampled from DSDP/IPOD Site 450 in the Parece Vela Basin, Philippine Sea and analyzed by electron microprobe. The ashes are interpreted as eruptive products of the adjacent West Mariana arc system between 25 and 14 Ma B.P., and have compositions between basaltic andesite and rhyolite, and rarely, boninite. ‘Continuous’ chemical trends appear to reflect mixing of mafic and silicic magmas. ‘Discontinuous’ trends between these end-members are relatively few, and are consistent with ‘liquid lines’ produced by fractional crystallization. Andesitic tephra become progressively richer in MgO and CaO through the middle Miocene, while boninite appears towards the end of the sequence, between 14 and 15 Ma B.P. Coeval rhyolitic glasses become richer in K 2 O and Na 2 O, with maximum concentrations at about 15 Ma B.P. Chronologic changes in fractionation type and composition of parent magmas are interpreted to reflect the subaerial volcanic evolution of the West Mariana arc. The appearance of boninite is believed to signal early stages of arc sundering, and corresponds temporally with regional uplift of the sea floor above the carbonate compensation depth, precursor to a new pulse of back-arc spreading.