Steven L. Dorobek

Petrography, Geochemistry, and Origin of Burial Diagenetic Facies, Siluro-Devonian Helderberg Group (Carbonate Rocks), Central Appalachians

Petrographic and geochemical studies of pore-filling cements and replacement products in the Helderberg Group (Upper Silurian-Lower Devonian, central Appalachians) document the diagenetic history of these rocks. Shallow-ramp skeletal limestones and buildups were partially lithified by marine cements. However, most pore-filling cement formed under shallow (<300 m) to deep burial (300-4,000 m) conditions. Regional cathodoluminescent zonation patterns in early, calcite cements indicate meteoric ground waters were involved in shallow burial cementation. Early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone, that pass downdip into subzoned dull cements (interlayered bright and dull laminae), and then into nonzoned dull cement in basinward limestone. The regional distribution, stable isotopic compositions, and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip (at least 150 km). Helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones. Late, void-filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4,000 m. Deep burial pore fluids were dilute to saline Na-Ca-Cl brines (from primary fluid inclusion data) with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields. Void-filling dull calcite cements occluded nearly all remaining porosity before late Paleozoic fracturing and deformation. Latest diagenetic phases include dolomite, fluorite, silica, and fracture-filling dull calcite. This study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution.

Sediment supply, tectonic subsidence, and basin-filling patterns across the southwestern South China Sea during Pliocene to recent time

Sediment flux to southwestern parts of the South China Sea (SCS) during late Cenozoic time reflects contributions from eastern Tibet, western Borneo, and smaller drainages of central Indochina, Vietnam, the Malay Peninsula, and western Indonesia, although little work has been done to evaluate the significance of each source. Regional seismic-reflection data and well logs from the southwestern SCS were used in this study to evaluate sediment flux and dispersal across the area. Regional seismic-stratigraphic patterns across the southwestern SCS, however, show that Pliocene to Recent sediment accumulation within individual basins was also strongly influenced by long-term changes in tectonic subsidence. More updip basins (e.g., Malay, Cuu Long, and West Natuna basins) became filled after Miocene inversion and an abrupt slowing of tectonic subsidence. Once they became filled, sediment could bypass the updip basins. In contrast, the eastern part of the Nam Con Son Basin (NCSB) has experienced much greater subsidence since early Miocene time and continues to receive sediment that bypasses the updip basins. The paleo-Mekong River and a second depositional system with probable head-waters on the Malay Peninsula began supplying large volumes of sediment to the NCSB during late Miocene and Pliocene time, respectively. Filling of updip basins allowed Pliocene to Recent fluvial and shelf facies to shift progressively eastward across the southwestern SCS. This study shows that Pliocene to Recent sediment dispersal and paleogeographic evolution of the southwestern SCS are as strongly influenced by subsidence patterns as they are by sediment supply from continental drainage systems.

Supply of Allochthonous Sediment and Its Effects on Development of Carbonate Mud Mounds, Mississippian Lake Valley Formation, Sacramento Mountains, South-Central New Mexico, U.S.A.

ABSTRACT Carbonate mud mounds in the Mississippian Lake Valley Formation, New Mexico, can be classified according to their external morphology, internal growth phases, and paleogeographic location along the Lake Valley ramp. Most of the Lake Valley mounds grew during a complete third-order accommodation cycle (Alamogordo-Nunn-Tierra Blanca interval; 3-6 Myr duration), which directly influenced mound biota, facies types, and the physical interactions between the mounds and sediment gravity flows that transported skeletal sediment around and over the mounds during the regressive phase of the cycle. Lenticular mounds are found in updip parts of the preserved Lake Valley ramp, which largely consists of outer ramp facies. Most lenticular mounds grew exclusively during deposition of the transgressive Alamogordo Member, when outer parts of the ramp were somewhat sediment starved. Coarse-grained carbonate gravity-flow deposits (Nunn and Tierra Blanca members) typically onlap lenticular mounds and only locally interfinger with mound-core facies. These relationships indicate lenticular mounds were buried by carbonate gravity-flow deposits, which accumulated much faster than mound cores could aggrade. The sediment gravity flows constructed shingled lobes, sand sheets, and oblique clinoforms and were erosive at times, which also probably helped to shape lenticular mounds. Transitional mounds are typically located downdip from the lenticular mounds and had an initial lenticular growth phase, which was followed by aggradational growth of mound cores. Aggradational mound cores were locally modified by submarine erosion or failure, which created sharp stratal truncation surfaces that were later onlapped by gravity-flow deposits. After the aggradational phase and stratal onlap, mounds with remnant seafloor relief served as substrates for continued growth during deposition of the Tierra Blanca Member. This final growth phase in transitional mounds records minor aggradation and significant, typically asymmetric, lateral accretion, which suggests that: (1) the mounds had grown to some accommodation limit (possibly storm wave base), and (2) the complex dispersal and accumulation patterns for the gravity flows also influenced how mound cores accreted laterally. For example, the upcurrent sides of some transitional mounds were buried preferentially by onlapping Tierra Blanca gravity-flow deposits, whereas their downcurrent sides locally accreted for significantly greater distances and with very different stratal geometries. Hemispherical mounds are located farthest downdip on the Lake Valley ramp. They are the largest mounds and contain all three (lenticular, aggradational, and accretionary) growth phases. Later growth phases of hemispherical mounds have steeply dipping mound core-to-flank relationships with abrupt facies transitions. Because of their distal position along the Lake Valley ramp and greater distance from the progradational sand bodies, the hemispherical mounds were the dominant sources for coarse-grained carbonate sediment to adjacent non-mound environments during Nunn and Tierra Blanca time. Thus, the distance of lateral accretion and overall morphology of hemispherical mounds were not affected greatly by allochthonous sediment from updip parts of the ramp.

Stable isotopic composition of meteoric calcites: Evidence for early mississippian climate change in the mission canyon formation, Montana

Abstract The Lower Mississippian Mission Canyon Formation of central to southwestern Montana was deposited under dominantly semiarid to arid climatic conditions during Osagean to early Meramecian times. Following deposition, a pronounced climatic shift to more humid conditions occurred during middle Meramecian times. This climatic change is indicated by extensive, post-depositional karst fabrics and in the stable isotopic composition of early, meteoric calcite cements and diagenetically altered sediments. Early meteoric calcite cement in Mission Canyon limestones is generally nonluminescent and fills intergranular and fenestral porosity. Petrographic data indicate that this cement formed during intermittent subaerial exposure of the Mission Canyon platform during Osagean times. This initial generation of meteoric calcite cement has δ18O values from −8.1 to −2.6%. PDB. These data, and the oxygen isotopic values from nonluminescent skeletal grains and micrite in host limestone indicate that Osagean meteoric water may have had δ18O values as low as −6.0%. SMOW. A second generation of petrographicalty similar, but isotopically distinct, calcite cement fills biomolds and porosity within solution-collapse breccias in the Mission Canyon Formation. This cement generation postdates earlier nonluminescent Osagean calcite cement and is volumetrically most abundant near the top of the Mission Canyon Formation. δ18O values from these cements and from nonluminescent lime mudstone clasts and matrix in solution collapse breccias range from − 13.8 to − 8.2%. PDB. These data indicate that Meramecian meteoric water may have had δ18O values as low as − 12.0%. However, a higher-temperature burial overprint on the δ18O values of the calcite cement cannot he ruled out. The more positive δ18O values of the Osagean calcite components probably indicate warm and arid conditions during short-term [104(?) yr] subaerial exposure along intraformational sequence and parasequence boundaries. The more negative δ18O values from Meramecian calcite components and the extensive karst associated with the post-Mission Canyon unconformity may have developed because of cooler and more humid climatic conditions and possible rain-out effects during middle Meramecian times. A dramatic shift towards cooler and more humid climatic conditions may be coincident with the onset of major continental glaciation in the Early Carboniferous. The post-Mission Canyon unconformity has been attributed to a major fall in sea level that may have glacio-eustatic origins. Growth of continental glaciers during a time of global cooling would have caused migration of polar fronts further toward the paleoequator. These polar fronts in turn, would have pushed moist, mid-latitude weather systems toward the paleoequator, resulting in cooler, more humid conditions in low-latitude settings during “icehouse” times.

Effects of ancient porosity and permeability on formation of sedimentary dolomites: Devonian Jefferson Formation (Frasnian), south-central Montana

Petrographic and geochemical evidence indicates that multiple dolomitization and dolomite stabilization events affected the Devonian Jefferson Formation (Frasnian) in south-central Montana. Several types of dolomite occur, defined by cathodoluminescence: nonzoned, dully luminescent subhedral-anhedral mosaics (most common), euhedral nonzoned and zoned dolomites, zoned dolomite cements, and irregularly luminescent dolomites (dully luminescent with irregularly luminescent regions). The irregularly luminescent fabrics probably represent partial replacement of early dolomite phases with later dolomite phases. Nonzoned, Ca-enriched, euhedral dolomites occur in calcite-cemented, coarse-grained limestone layers. These permeable layers probably were conduits for early meteoric waters, that occluded porosity in the limestones and prevented later dolomite stabilization. Irregularly luminescent dolomites are interpreted as intermediate fabrics in the dolomite stabilization process. Later calcite cements which occlude intercrystalline porosity prevented further dolomite replacement. Total recrystallization of remaining dolomites and formation of final dully luminescent mosaics occurred prior to brecciation and stylolitization.