Eugene C. Rankey

Relations between relative changes in sea level and climate shifts: Pennsylvanian–Permian mixed carbonate-siliciclastic strata, western United States

Study of Pennsylvanian (Virgilian)–Permian (Wolfcampian) strata of the western United States (parts of Idaho, Utah, New Mexico, and Kansas) reveals complex relations between relative changes in sea level and shifts in climate. Relative falls in sea level are indicated by subaerial exposure of subtidal sediments and fluvial incision through subtidal sediments. Changes in the character of pedogenesis and sediment transport, deposition, and supply, and the variable distribution of climatically sensitive lithofacies within cyclothems are interpreted to reflect shifts in climate during deposition of each cyclothem. The majority of stratal relationships documented herein from a number of long-term climatic, sedimentologic, and shelf settings are consistent with a hypothesis that relative highs in sea level are accompanied by more seasonal conditions, whereas relative lows are characteristically more arid. Uncommon deviations are present, however, and may be due to (1) shoaling caused by sedimentation or tectonic uplift, independent of a eustatic-climatic signal; (2) complex sedimentologic or geomorphic responses to climate change; (3) leads, lags, or thresholds in either glacial advance and retreat or the climatic response to such changes. This study shows that coupled climatic shifts and changes in sea level exert a pronounced, but varied, influence on stratigraphic architecture in depositional settings across late Paleozoic ramps. The proposed climatically enhanced cyclic and reciprocal sedimentation model predicts that during a single eustatic fall, updip areas have features indicating a more seasonal or humid climate that are overlain or overprinted by characteristics suggesting more arid climate. In downdip areas that were not subaerially exposed until the late stages of a eustatic fall, only more arid climatic features are represented in the terrestrial or subaerial exposure features.

Holocene ooids of Aitutaki Atoll, Cook Islands, South Pacific

Although oolitic sands are widespread throughout the Phanerozoic carbonate rock record, and they are abundant in some modern shallow-marine carbonate systems, recent ooids have not been recognized from any locality in the Pacific Basin. On Aitutaki Atoll (Cook Islands, South Pacific), oolitic sands occur in water depths of 2.1–6.3 m within the lagoon and, less abundantly, on shallower reef aprons. Most ooids are between 150 and 300 μm in diameter, have between 1 and 25 concentric to irregular laminae, and occur with peloids and skeletal grains. Cortical laminae include Mg-calcite crystals with radial, tangential, and random orientations. The occurrence and distribution of oolitic sands on Aitutaki are facilitated by a convergence of hydrodynamic and chemical factors. Wave-driven currents, combined either with flow separation over a sharp increase in depth at the reef apron–lagoon interface or with oceanward flow in the lee of islands, serve to transport ooids without flushing them from the system. Here, pH, alkalinity, and carbonate supersaturation are at some of the highest levels in the tropical Pacific. The restricted occurrence of Holocene oolitic sands in areas of the Pacific and Atlantic Basins with elevated pH and total alkalinity reflects the importance of carbonate saturation thresholds in limiting the spatial distribution of these sands in modern shallow-marine settings. These recent examples are consistent with interpretations of the importance of these variables in determining the distribution, abundance, and cortical mineralogy of oolitic sands throughout the Phanerozoic stratigraphic record of carbonate accumulation.

Relations between geomorphic form and sedimentologic-stratigraphic variability: Holocene ooid sand shoal, Lily Bank, Bahamas

Although the linkages among surface sediments, geomorphic forms, and hydrodynamics in Holocene ooid tidal sand shoals have been evaluated recently, how these factors are reflected in the geomorphic evolution and stratigraphic record of shoals is less constrained. Yet, such understanding is essential to developing meaningful predictive conceptual models of three-dimensional architecture of ancient reservoir analogs. Integrating remote-sensing imagery, high-frequency seismic data, and core characterization from Lily Bank, a modern tidally dominated Bahamian ooid shoal in which sedimentologic processes are well documented, reveals the stratigraphic record of geomorphic change. An irregular, gently dipping rocky surface (interpreted as the top Pleistocene) with no pronounced topographic high underlies the Holocene oolitic succession. A 6-m (20-ft)–thick poorly sorted, gravelly muddy sand with few ooids overlies this basal surface. This lower interval is overlain by sand with an upward increase in proportion of ooids, sorting, and grain size. The uppermost unit, present only under active bar forms, is well-sorted oolitic medium sand with accretionary foresets. Sediments vary stratigraphically and geomorphically; the lower unit is finer and less well sorted than the upper units, and in the oolitic upper unit, sediment size and sorting on bar crests are distinct from bar flanks. Collectively, these results suggest that a marked antecedent bump is not necessary for occurrence of ooid shoals and that the stratigraphic record of analogous ooid shoal systems may preserve clues of geomorphic position, as well as geobody size and orientation.

Holocene Indian Ocean tsunami history in Sri Lanka

Sediment cores from Karagan Lagoon in southeastern Sri Lanka retrieved deposits from the A.D. 2004 Indian Ocean tsunami and older similar deposits that provide evidence for a tsunami 2417 ± 152 cal. (calendar) yr B.P. to 2925 ± 98 cal. yr B.P., and for six tsunamis between 4064 ± 128 cal. yr B.P. and 6665 ± 110 cal. yr B.P., a period for which the sediment record appears continuous. Radiocarbon dating indicates that the recurrence interval is variable, ranging from 181–517 yr to 1045 ± 334 yr, with a mean recurrence interval of 434 ± 40 yr during the ca. 4000–7000 cal. yr B.P. continuous interval. Assuming that these tsunamis were generated by giant earthquakes along the Sumatra-Andaman subduction zone, a reasonable assumption for this far-field transoceanic location, this record extends the giant-earthquake history for the Indian Ocean region. The longest recurrence interval of more than 1000 yr implies that earthquakes along the subduction zone may reach twice the size of the 2004 earthquake.

Tidal Sands of the Bahamian Archipelago

Tidal sands consisting entirely of carbonate sediments are ubiquitous in the Bahamian archipelago. These sands include a diversity of sediment types, including ooids, peloids, and skeletal fragments. Sands transported by tides, waves, and currents create barforms in tidal sand complexes with a range of shapes and sizes. These features are shaped by, and in turn modify, tidal currents that move on and off the shallow platforms; waves and wave-driven currents play a subordinate but locally important role in their genesis and architecture. Collectively, barforms make up shallow shoal complexes. These shoal complexes are focused in areas with elevated tidal currents (locally in excess of 200 cm/s) near platform margins, and can exceed 10 km in width. The diversity of barforms and shoal morphology evident in Holocene examples is reflected in the stratigraphic record of numerous ancient tidal sand shoals, with preservation favored by the early cementation ubiquitous in these carbonate systems.

Preserved pedogenic mineral magnetic signature, pedogenesis, and paleoclimate change: Pennsylvanian Roca Shale (Virgilian, Asselian), central Kansas, USA

Abstract This study focuses on the field characteristics and mineral magnetism of four stacked paleosols in the Upper Pennsylvanian (Virgilian, Asselian) Roca Shale exposed near Manhattan, Kansas, USA. The Roca Shale paleosols (PI at base, P4 at top) include a protosol (P1), argillic calcisol (P2), calcic argillisol (P3), and calcic vertisol (P4). Soil climatic changes during the development of each soil profile are suggested by cross-cutting relationships and superposition observations, including: (1) clasts of a lower, vertic part are incorporated in an upper, calcic horizon, and (2) carbonate glaebules have argillans covered by (hematitic) ferrans. Decreasing carbonate content, more strongly developed of vertic properties and argillans, and more mature profiles in successive paleosols are consistent with an interpretation of a longer-term soil climate shift from more arid to more seasonal or humid during development of the four paleosols. Several observations are consistent with a preserved pedogenic mineral magnetic signature. In particular: (1) each type of soil horizon (B, Bt, Bk, BE, and C), within and among paleosols, contains consistent magnetic susceptibility (χ), natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) signatures, different from other horizons; (2) similar redox trends are indicated by magnetic mineralogy and uranium concentrations; (3) observed magnetic character and uranium concentrations are compatible with those expected in gleyed and oxidized intervals, as defined by field observations; (4) pedogenic carbonate glaebules and adjacent pedogenically-altered siliciclastic strata preserve the same χ trends; and (5) paleosols contain single domain (SD) magnetite and some maghemite, similar to many modern soils. Comparison with other Paleozoic vertisols indicates that the Roca Shale paleosols are characterized by more complex pedogenic and mineral magnetic attributes. This variability in the Roca Shale paleosols is probably related to glacial-interglacial climatic variability during pedogenesis. The results of this study show that coupled field observation and mineral magnetic characterization of other pre-Quaternary paleosols potentially will yield significant paleoclimatic and pedogenic information.

Holocene Carbonate Tidal Flats

Carbonate tidal flats of the Bahamian archipelago and the Arabian Gulf have served as important analogs for interpreting and understanding ancient tidal flat systems. Geomorphic associations include well-zoned subtidal, intertidal, and supratidal environments and their deposits, each with distinctive associations of biota and biologic and physical sedimentary structures. Although they include broadly similar facies associations in each environment within and between tidal flats, the occurrence and distribution of specific facies across landscapes differs markedly between tidal flats. Depending on the details of climate, tidal amplitude, regional setting and energy level, Holocene carbonate tidal flats include systems penetrated by numerous sinuous channels with adjacent levees and ponds, areas with broad, flat progradational intertidal and supratidal plains, and regions with shorelines that appear to have abruptly stepped oceanward or eroded. Stratigraphically, each different type of tidal flat includes a shallowing-upward facies succession, although in many areas, a basal transgressive unit is present.

Geostatistical facies modeling trends for oolitic tidal sand shoals

To assess prospective modeling trends for oolitic tidal sand shoals and explore potential patterns of reservoir heterogeneity, this study examines, quantifies, and models the cycle-scale architecture of the Holocene mobile oolitic tidal sand shoal complex at Schooner Cays, Bahamas. Process-based stratigraphic trends are captured in quantitative, geocellular models of the shoal from analyses of satellite imagery; two-dimensional, high-frequency seismic (chirp) data; and sediment cores. Data show that longitudinal tidal sand ridges extend up to 8 km (5 mi) along depositional dip, gradually transforming bankward into channel-bound, compound barforms consisting of linear, parabolic, and shoulder bars. These bars terminate into a laterally extensive (10 km [6 mi]), strike-elongate sand sheet. Each bar type includes distinct internal architecture, grain size, and sorting related to feedbacks among hydrodynamics, geomorphology, and sedimentology. Building on these data and concepts from the Holocene accumulations, this study demonstrates a methodology for quantifying and validating probabilistic stratigraphic trends prior to their inclusion in stochastic-based facies modeling algorithms. Inclusion of statistically robust facies probability volumes during truncated Gaussian simulation generated ordered and geologically accurate facies distributions relative to bar-crest centerlines, water depth, and geomorphic position. Petrophysical models that incorporate facies-specific porosity, permeability, and water saturation functions display pronounced cycle-scale heterogeneity that could provide insights into variable production rates and poor sweep efficiency commonly encountered during development of analogous oolitic reservoirs.

ABSTRACT: Sizes, Shapes, and Patterns of Sediment Accumulations on a Modern Tidal Flat and Stratigraphic Implications: Three Creeks Area, Andros Island, Bahamas

Construction of realistic geologic and simulation models of subsurface reservoirs requires data on the geometry and continuity of flow units, baffles, and barriers, parameters commonly constrained by log, core, seismic, and production data. If the minimum horizontal dimensions of facies bodies is less than the typical well spacing; however, properties will not be accurately described using either deterministic or stochastic methods. In these situations, seismic or production data can provide insights, yet still may include ambiguous characterization.

Insights into Permian Pedogenesis and Climate: Magnetic Stratigraphy of Four Stacked Paleosols, Lower Permian (Wolfcampian) Roca Shale, Central Kansas: Abstract

Abstract This study focuses on the rock magnetic character of four stacked paleosols (P1 at base, P4 at top) in the Lower Permian (Wolfcampian, Council Grove Group) Roca Shale exposed near Manhattan, Kansas. Magnetic characterization of Quaternary soils has proven a valuable tool for evaluating climatic conditions and pedogenic processes, yet no systematic study of pre-Quaternary soils has been undertaken. The Roca paleosols are protosols, vertisols, calcic vertisols, and calcisols. Cross-cutting relationships and superposition principles suggest that macro- and micro-climatic conditions evolved as soil formation progressed. In particular: 1) in P2, clasts of the lower (vertic) part are entrained in the upper (calcic, leached) part suggesting a shift from subhumid to semiarid; and 2) in P3 and P4, carbonate nodules have argillic coats that are at times overlain by (hematitic) ferrans, suggesting climate transitions from semiarid to subhumid back to semiarid. Rock magnetic data provide further insight into pedogenesis and climate. A preserved magnetic signature is suggested by: l) distinct magnetic susceptability, natural remanent magnetization (NRM), anhysteric remanent magnetization (ARM) and isothermal remanent magnetization (IRM) in B, Bt, Bk, E, and R horizons; and 2) carbonate nodules and adjacent subangular blocky structure which preserve the same susceptability trends. Overall, susceptability ranges from <2.0 to 11.6 x 10{-8}m{3}/kg, but within P2, P3, and P4, there is a general upwards decrease. PI contains upwards increasing susceptability. These changes correspond to a shift in S-ratios (IRM[0.3T]/SIRM) ratios from ~0.9 to ~0.4, reflecting a possible change from magnetite to hematite upwards through the paleosol profile. NRM values range from almost 1 x 10{-8}`to 5 x 10{-5} Am{2}/kg. NRM/susceptability and SIRM/susceptability ratios (proxies for ferrimagnetic vs. paramagnetic contributions to susceptability) decrease from PI to P4 and the position of the highest ratio within each paleosol profile is lowered from the B (P1) to top Bt (P2) to base Bt (P3, P4) horizons. Rock magnetic data and field observations suggest that weathering intensity increased through time from the development of P1 to P3 (P3 represents the most leached and oxidized paleosol profile), but decreased during development of P4. The oxidizing conditions prevalent in upper soil horizons led to increased lessivage in these intervals and subsequent concentration of clays and iron oxides in lower, less oxidizing horizons. Gleyed intervals are characterized by (SD?) magnetite. The results of this study show that coupled field observation and rock magnetic characteristics of pre-Quaternary paleosols potentially will yield significant climatic and pedogenic information.