Abstract This chapter focuses on the processes, character, and significance of hyper-pycnal floods and their deposits (hyperpycnites) in the geologic record based on selected examples from modern environments, outcrops, sediment cores, and seismic reflection profiles. For bed-load- or suspended-load-dominated hyperpycnites, the rating curve of a flood event can be predicted and is represented by a power-law relationship between discharge and load. Because of reconcentration processes in marine waters, rivers other than those considered dirty (high sediment concentration) may form hyperpycnal flows over long distances on the sea floor. Hyperpycnal floods generally have a meteorologic origin, but catastrophic hyperpycnal floods can also occur because of dam outburst in the catchment area of a river. In some cases, catastrophic floods are associated with earthquakes or volcanic activity. Both in marine and lacustrine environments, frequent and powerful hyperpycnal flows develop meandering channel levee systems and more distal basinal fan lobes. Hyperpycnites are characterized by a coarsening-upward basal sequence formed during the rising limb of the flood (waxing flow) and a fining-upward upper sequence formed during the falling limb of the flood (waning flow). The lower sequence is not always preserved, so hyperpycnites may be difficult to distinguish from classical turbidites. The common occurrence of organic debris of continental origin at the base of deposits is an indicator of hyperpycnites. In glacial valleys of mountain ranges, a large amount of clastic material can be deposited in large valley lakes or in fjords by frequent hyperpycnal flows. Both in lacustrine and marine environments, large hyperpycnal floods, which are sometimes catastrophic, are documented in association with significant environmental changes such as deglaciation of the continent or volcano-glacier interactions (jokulhlaup).

Abstract Facies analysis performed for more than 10 yr in several lacustrine and marine basins allowed the distillation of a genetic and predictive facies tract of general application to the recognition of sustained turbulent (noninertial) hyperpycnal flow deposits. The facies tract is composed of three main genetic facies groups related to bed load (B), suspended load (S), and lofting (L) transport processes. Facies B (bed load) is the coarsest grained and relates to shear and frictional drag forces provided by the overpassing long-lived turbulent (hyper-pycnal) flow. Three main subcategories are recognized, termed B1 (massive or crudely bedded conglomerates), B2 (pebbly sandstones with low-angle asymptotic cross-stratification), and B3 (pebbly sandstones with diffuse planar lamination and aligned clasts). Facies S is finer grained and relates to the gravitational collapse of suspended load transported by the hyperpycnal flow. Four subfacies types are recognized, termed S1 (massive sandstones), S2 (parallel laminated sandstones), S3 (sandstones with climbing ripples), and S4 (massive siltstones and mudstones). Facies L (lofting) relates to the buoyancy reversal of the hyperpycnal flow provoked by lift of a less dense fluid (in this case, fresh water) typically found in marine and other saline-receiving basins. The finest materials suspended in the flow (very fine-grained sand, silt, plant debris, and mica) are lifted from the substrate and settle down forming silt/sand couplets of great lateral extent. Facies L develops only in marine/saline environments, whereas facies S3 and S4 are more common in lacustrine environments. Hyperpycnites are often very complex, showing internal erosional surfaces and gradual facies recurrences related to deposition from long-lived and highly dynamic (fluctuating) flows. This complex behavior results in the accumulation of composite beds having an internal facies arrangement that strongly departs from conventional facies models developed for surgelike flows. Facies B characterizes transfer zones, and its occurrence allows the prediction of sandstone deposits (facies S) basinward. Facies L is mostly developed in flow-margin areas. In marine settings, the reversing buoyancy effect (lofting) at the hyperpycnal flow margin will result in less lateral continuity of sandstone bodies compared with those related to resedimentation processes, with important consequences for hy-drocarbon reservoirs.

Abstract The mapping and analysis of ancient turbidite deposits using geograph-ically dependent sedimentologic indices (or proximality) determined by genetic facies relationships and grain-size variations were mostly applied during the early years of turbidite research. Nevertheless, the understanding that fine-grained deposits (thin-bedded turbidites) can also occur in proximal (interchannel) areas, and the impossibility of differentiating them from real distal deposits, revealed many inconsistencies in this approach to the development of turbidite fan facies models. More recently, a genetic facies tract has been proposed for hyperpycnal systems, which distinguishes between thin-bedded deposits accumulated in interchannel areas from those deposited on distal areas. The genetic facies tract proposed for hyperpycnal systems in marine settings comprises three main facies categories related to bed-load, suspended-load, and lofting processes, respectively. Bed-load-related facies are coarse grained and related to drag forces provided by the overpassing turbulent flow. Suspended-load-related facies are composed of fine-grained sandstones and originate from the gravitational collapse of the suspended load as the long-lived flow progressively wanes. Lofting-related facies are the result of the fallout of very fine-grained sands, silts, plant debris, and micas from lofting plumes mainly in flow-margin areas. Proximal (Pt) and lateral (Lt) facies indices have been defined based on the relative abundance of B, S, and L facies within a stratigraphic interval. The Pt and Lt indices are dimensionless numbers that range from 100 to 0. The Pt index measures how proximal a given stratigraphic section is with respect to the entire hyperpycnal system. The Lt index provides an indication of the stratigraphic lateral distance of a given section with respect to the flow axis. These genetic indices were applied to hyperpycnal deposits of the late Oligocene-early Miocene Merecure Formation, Maturin Subbasin, Venezuela. Results suggest that the Merecure Formation may be more extensive than previously considered with a main sediment source from cratonic areas located in the south and southeast. Index mapping also suggests a syntectonic accumulation during the Oligocene, which controlled the subaqueous topography and, thus, the distribution of sand bodies.

Abstract In the Miocene beds of Cabo Viamonte, deep-marine hyperpycnal channel levee complexes occur at the toe of the depositional slope in sigmoidal clinoforms. The hyperpycnal channel fill is composed of thick packages of transitional and recurrent (vertical and lateral) sedimentary structures without rheologic boundaries, with variable textural ranges, recurrence of inverse-to-normal grading, multiple internal reactivation surfaces, and abundant organic content. This complex internal arrangement allows recognition of three facies associations (FAs) in the hyperpycnal channel fill. The FA1 comprises the coarsest clastic fractions, composed of intraformational boulders, matrix- and/or clast-supported conglomerate, and gravelly sandstone, which results from the combination of inertial flows and turbidity currents generated during the circulation of hyperpycnal flows. The FA2 comprises conglomerate, gravelly sandstone, and coarse- to fine-grained sandstone that results from rapid aggradation from a hyperpycnal turbidity current capable of transporting gravel as bed load and particles up to the size of coarse sand in turbulent suspension. The FA3 comprises heterolithic deposits that result from progressive accumulation by traction and suspension processes of low-density hyperpycnal turbidity currents. The coarsegrained lower channel fill is interpreted as a single sedimentation event. This is suggested by multiple internal erosion surfaces without lateral continuity, the lack of lithologic contrasts between sediment below and above the reactivation surfaces, and the absence of biogenic structures. On the contrary, the fine-grained upper channel fill, deposited mainly during the waning stages of the hyperpycnal flow, may involve more than one sedimentation event. This interpretation is supported by the presence of isolated beds with low-diversity trace fossils within the associated mudstones.

Abstract This chapter is about an almost uninterrupted exposure of the Jurassic Los Molles Formation in the Arroyo La Jardinera region in the southern Neuquén Basin (western Argentina). It is based on the analysis of high-resolution satellite images linked with extensive field data. Its main goal is to present a conceptual model for the studied turbidite system capable of explaining how sand and gravel were brought onto and removed from the shelf margin and how these coarse-grained sediments were transported along and deposited on the slope and basin plain. The facies tract ascribed to this turbidite system represents high-frequency lowstand system tracts. From proximal to distal, it includes the record of fluvial channels incised into mid- to outer-shelf deposits, turbidite channels scoured on slope strata, turbidite lobes resting on slope rise facies, and poorly gravity flow strata were assigned to short-lived (surges) moderately efficient turbidity currents derived from the almost continuous failure of delta-front deposits (sand and gravel) during ordinary fluvial floods.

Abstract The Rio Guache Formation represents deep-marine deposits accumulated within a diachronic foreland basin setting formed as a result of the Paleo-gene oblique collision between the transpressional front of the Caribbean plate and the Cretaceous to Paleogene passive margin of northern South America. This study attempted to provide the first comprehensive sedimentologic anal-ysis of the Rio Guache Formation in the Guaramacal area. Based on the study of outcrops and samples, eight facies have been identified as composing the Rio Guache Formation, five of them gravelly and three sandy. The facies analysis and the interpretation of the processes of transport and deposition for each of the different facies indicate that they are the products of the progressive transfor-mation of gravity flows within the basin. The distinct assemblages of facies, the high percentage of coarse-grained sediments, the poor textural sorting, and the minor development of sedimentary structures indicate that these gravity flows were short-lived and poorly efficient. They could be the product of episodic mass flows and sliding induced by the high tectonic activity in western Venezuela during the late Paleogene. No direct evidence of a hyperpycnal origin for these coarse-grained strata exists.

Abstract The Eocene Central Basin on Spitsbergen, Neogene offshore eastern Vene-zuela, and offshore northern Sakhalin are high-sediment supply systems that do not obey standard rules of sequence stratigraphy. These studies show that high sedimentation rates are capable of filling accommodation space created during transgression and highstand. The thick successions of coastal plain deposits in these basins consist of fluvial and estuarine deposits and are inter-preted to result from rapid increases and cutoffs in fluvial sediment supply gen-erated by tectonic pulses and climatic cycles. We suggest that the areal positioning of fluvial input points along a margin will give different shelf geometries. A shelf may be fluvial dominated or wave or tide influenced at the same time, which together with multiple fluvial input points creates lateral variations in the depositional architecture. The results from any studied two-dimensional section will therefore be very dependent on which part of the margin is studied, and changes in depositional architecture might be in-correctly interpreted as basinwide sea level changes. Prolonged or periodically high sedimentation rates result in progradation of the fluvial-deltaic system toward the shelf edge. With high sedimentation rates, the sediments reaching the shelf edge are prone to collapse, creating failure back to the contemporary shoreline. The collapse scars will act as conduits, and the fluvial system will be capable of overriding the shelf edge and transporting deposits to the deep water by hyperpycnal flows.

Abstract Hyperpycnal flow deposits and associated facies in upper Cretaceous res-ervoirs of the Magallanes Formation in the Campo Boleadoras-Estancia Agua Fresca-Puesto Peter area of southern Patagonia, Argentina, are docu-mented based on subsurface data. The study represents one of the first ichnologic characterizations of a deltaic system dominated by hyperpycnal processes. The fluvial-deltaic system was sourced from uplifted areas located in Central Patagonia and the Río Chico High and prograded toward the south and southeast. In-tegration of sedimentologic and ichnologic data allows establishing proximal-distal trends within a sediment transport system. The bulk of sandy hyperpycnal lobe deposits consists of coarse- to fine-grained sandstones that are either structureless or display a subtle parallel lamination that is commonly delineated by abundant plant remains. Vertical grain-size changes reflect flow fluctuations, with coarsening- and fining-upward intervals indicating waxing and waning flows, respectively. The concentration of plant remains indicates phytodetrital pulses in connection with direct fluvial discharges. These deposits are unburrowed to sparsely bioturbated, containing the Thalassinoides ichnofabric, which records opportunistic colonization during times of decreased sedimentation rate. However, the pervasive laminated fill of the burrows may reflect relatively high sedimentation rates, most likely because of suspension fallout of fine-grained material. The extreme compaction suggests burrow emplacement in a water-saturated soft substrate that underwent compaction subsequent to the bioturbation event. Associated heterolithic intervals contain the Planolites-Teichichnus ichnofabric, which characterizes marginal areas with respect to the hyperpycnal sand-rich lobes and times of quiescence between flows. During times of reduced sediment supply, material was reworked by wave processes, and hypopycnal conditions were dominant. The Thalassinoides-Teichichnus ichnofabric records colonization of these wave-reworked sandstone units, whereas the Terebellina-Phycosiphon ichnofabric reflects stable conditions that allowed intense bioturbation and the establishment of a moderately diverse benthic fauna. Fully marine offshore deposits are characterized by the Teichichnus-Phycosiphon ichnofabric, which display total biogenic reworking, high ichnodiversity, and a complex tiering structure.

Abstract Combined ichnologic and sedimentologic analysis of Miocene deep-marine hyperpycnites in the Austral foreland basin, Tierra del Fuego, Argentina, allows recognition and characterization of different trace fossil assemblages from proximal to distal within this depositional system. In proximal and middle settings, hyperpycnal channel and inner levee deposits contain trace fossils in climbing-ripple and parallel-laminated sandstone facies; dwelling structures of suspension-feeding organisms (e.g., Diplocraterion ) dominate. In outer levee depos-its, fine-grained heterolithic intervals display a suite of trace fossils related to op-portunistic producers composed of concentrically laminated burrows of detritus or deposit feeders and also locomotion and grazing traces (e.g., Protovirgularia, Gordia ). The general low intensity of bioturbation and the dominance of structures pro-duced by opportunistic organisms suggest a stressful environment probably be-cause of episodes with high sedimentation rates and fluctuating salinity. In addition, in the middle slope setting, escape structures of imported bivalves occur in isolated sandstone bodies that accumulated at the base of the depositional slope.

Abstract The Pliocene B4 sandstone is an important gas-bearing reservoir in the Oil-bird field, Columbus Basin, southeast Trinidad. The B4 sandstone is dominated by fine-grained, massive, and parallel-laminated sandstones inter-bedded with thinly laminated siltstone and very fine-grained sandstones deposited in a shelfal setting. A detailed sedimentologic study of the B4 reservoir was performed by integrating 60 ft (18.3 m) of core, 3770 ft (1149 m) of borehole image data, open-hole logs, mud logs, and biostratigraphic data. A total of 12 sedimentary facies were described and interpreted on the basis of sedimentation processes using a genetically oriented facies analysis approach. Six facies associations were identified based on the gamma ray (GR) pattern and the vertical facies association. Core data show mostly massive sandstones with very little to no bioturbation. These massive sandstones commonly alternate with intervals having diffuse lamination, which appear as a recurrent feature within the massive bodies. The tran-sitional recurrence of massive and parallel-laminated sandstones indicates velocity fluctuating and sustained turbulent flows. Sedimentologic evidences suggest that the origin of the B4 sandstone could be related to the paleo-Orinoco River-related product of turbidity (hyperpycnal) outflows that extended into the Columbus Basin. The associated occurrence of plant debris favors the interpretation of a direct fluvial supply by rivers in flood stage (hyperpycnal systems). The lateral correlation of facies associations throughout seven wells allowed the identification of six depositional units, named from base to top, A, B, C, D, E, and F. Facies maps for the B4 reservoir have been developed and were constrained by paleocurrent data extracted from image data. The B4 reservoir sand shows a progradational pattern reflecting the infill of a fault-controlled depocenter. Paleoflow data indicate axial transport roughly parallel to the main fault system, indicating that faults were active and controlled the accommodation space. The proposed hyperpycnal depositional model will introduce substantial changes for the prediction of the geometry and position of sandstone accumulations. The hy-perpycnal model predicts the occurrence of sand accumulations in the lower parts of the paleolandscape, whereas the higher parts of the basin (margins) are characterized by fine-grained sediments. Thus, this new depositional model could represent a drastic shift in the prospectivity guide for new exploration plays.

Abstract An outgrowth of the 2008 AAPG Hedberg Conference on Sediment Transfer from Shelf to Deep Water, Studies in Geology 61 was designed specifically to explore the growing interest in hyperpycnal and associated flows and hyperpycnites as significant contributors to the deep-water sedimentary record. The topic of hyperpycnal flows and their deposits, hyperpycnites, has recently emerged as the latest in a long list of hotly debated topics on deep-water sedimentary processes, environments, and deposits. This collection of chapters offers important new insights into the sediment delivery system to deep-marine waters.

Abstract Coastal cliffs in north Taranaki, western North Island, New Zealand, provide excellent exposures of a deep-water clastic depositional system of Miocene age, within which a wide range of reservoir facies and sedimentological features are represented. The overall succession is progradational. It comprises heterolithic but generally sand-dominated basin-floor to base-of-slope strata, deposited in middle-to-lower bathyal water depths (Mount Messenger Formation); these are overlain by siltstone-dominated slope strata deposited in middle- to uppermost-bathyal water depths (Urenui Formation). Urenui siltstones are intermittently cut by channels infilled with a variety of lithologies. These Miocene rocks are generally uncemented and only moderately indurated, and exposures are relatively unweathered and clean, owing to washing by waves during high tides.

Abstract At Pukearuhe Beach, the uppermost part of the sand-dominated Mount Messenger Formation is exposed, together with an apparently gradational, upwards transition into the overlying siltstone-dominated Urenui Formation (although the contact is offset by faulting). Magnificent cliff exposures extend left and right from Pukearuhe Road end and continue as far as Whitecliffs, 4 km (2.5 mi) to the north. The Mount Messenger Formation at Pukearuhe Beach comprises thin- to medium-thick interbeds of fine- to very fine-grained sandstone and siltstone. Broad scour-and-fill structures and interleaving channel and channel-overbank deposits are present in several places. The stratigraphie position and depositional architecture suggest deposition on a series of coalescing fan aprons at the base of the slope. These sediments are inferred to be comparable to Mutti Type III turbidites ( Mutti, 1985 ). Thickest sections viewed from the air display several broadly compensating, fining-upward cycles (interpreted as fifth-order), some of which appear to show moundlike geometries and bidirectional downlap.

Abstract Gas from the Pennsylvania Jackfork Group is generally considered to be trapped in large structures and to be produced from associated fractures. In outcrop, fractures are well developed in brittle, quartz-cemented sandstones. However, some outcrops also reveal the presence of friable and poorly cemented sandstones, which if present in the subsurface, could also comprise good reservoirs. Identifying these two different reservoir types (fracture vs. matrix porosity) can be challenging in structurally complex areas where there are little or no seismic profiles and only conventional well logs. Outcrop studies have shown that the Wildhorse Mountain Formation of the Jackfork Group is mainly composed of deep-water sheet and channel-fill sandstones. Each of these classes has distinctive features that allow their differentiation from conventional core, dipmeter, or borehole image logs, and sometimes conventional well logs. Sheet sandstones tend to be interbedded with shale to give a relatively low net sand; beds are laterally continuous for long distances, and stratigraphic dips are uniform and of relatively low angle. channel-fill sandstones tend to be thick-bedded, with higher net sand, lenticular external geometry, and variable stratigraphic dips. These patterns are readily identified on borehole image and dipmeter logs. Outcrop and subsurface studies in eastern Oklahoma have indicated the sheet sandstones are highly quartz-cemented, moderately to well sorted, with little primary porosity. channel-fill sandstones are more poorly sorted, contain clay, and are moderately to poorly cemented. Porosity in the sheet sandstones tends to be fracture-dominant and that in the channel sandstones tends to be matrix-dominant. Using these outcrop and well log criteria, it is possible to differentiate these two different reservoir types in the subsurface.

Abstract This paper presents the results of data collection, analysis, and integration to build a 3D geologic model of an outcropping leveed-channel complex. Data is from more than 120 standard measured stratigraphic sections, behind-outcrop drilling/logging/coring, ground-penetrating radar and electromagnetic induction surveys, and 2D shallow seismic reflection acquisition. This leveed-channel complex, which is part of the Dad Sandstone Member of the Cretaceous Lewis Shale, Wyoming, consists of ten channel-fill sandstones, confined within a master channel. The complex is 67m (200ft.) thick, 500m (1500ft.) wide, and has a net sand content of approximately 57 percent. Individual channel-fills are internally lithologically complex, but in a systematic manner which provides a means of predicting orientation and width of sinuosity. Although it has not been possible to completely document the three-dimensionality of this system, the 3D model that has evolved provides information on lithologic variability at scales which cannot be verified from conventional 3D seismic of subsurface analog reservoirs. This vertical and lateral variability can provide realistic lithologic input to reservoir performance prediction. An outcome of this study has been knowledge gained of the extent of manipulation required to obtain the spatially correct geometry and architecture of strata when integrating outcrop and shallow, behind-outcrop data sets.

Abstract Sea cliff exposures, conventional and high resolution seismic profiles, conventional oil and gas field boreholes, and shallow behind-outcrop core holes along the west coast of the Taranaki Basin, North Island, New Zealand, afford an excellent opportunity to examine the inter-relationships of scales of heterogeneity within slope channel-levee-overbank and toe-of-slope deep water sediments. Here, the Taranaki sea cliffs, are up to 240 m high and exhibit over 25 km of nearly continuous exposure. Across the spectrum from seismic profile to microscopic and instrumental analysis, detectable features that affect reservoir heterogeneity range across at least 13 orders of magnitude from greater than 10 4 m to 10 -9 m. Using the exposures along the Taranaki Coast, it is possible to assess the extent and detectability of the various scales of lithologic heterogeneity and to estimate their effects on analogous intervals in the subsurface. Bed/bedset heterogeneity ratios (calculated as vertical distance divided by horizontal distance of a given parameter), as detected by the tools mentioned above, cluster in two domains. Because of their limited radius of investigation, borehole logs have ratios between 10 0 and 10 -1 , whereas outcrop, seismic, and log profiles, which are based on a horizontal arrangement of data, generally show ratios between 10 -1 to 10 -3 . Because of their design limitations and strengths, each of the tools sees a different scale of lithologic or bedform heterogeneity. Data sets such as that available from the Taranaki cliffs section are invaluable for constraining estimates of reservoir heterogeneity in subsurface models.

Abstract Detailed geologic mapping of the Pennsylvanian Jackfork Group turbidites in the DeGray Lake area of Arkansas has provided a 3-D geologic model of a mile-long, steeply dipping turbidite succession that is separated into east and west fault blocks by a zone of strike-slip faults. By scaling-up the stratigraphy into four reservoir zones and by choosing two topographic ground surface elevations to represent an unconformity top-seal and an oil-water contact, this outcrop can be considered a pseudoturbidite “reservoir,” amenable to production simulation. “Reservoir” performance simulation was conducted in 3-D, three-phase black oil mode, using vertical- and horizontal-well drilling scenarios for both water drive and depletion drive cases. The results demonstrate the ability to perform simulation on an outcrop to visualize, directly examine, and, hence, better understand the geologic controls on the performance of analog oil and gas reservoirs.

Abstract To bridge the sometimes large conceptual gap between what is observed in outcrop and what occurs in the subsurface, outcrop information ideally should be placed into the more familiar format used for exploration and development, that is, well logs, seismic, cross sections, and 3-D models. Borehole imaging logs from behind an outcrop, when calibrated to the outcrop (and core), can be particularly useful for identifying image log criteria that can be used for predicting stratigraphic (and structural) features away from the borehole, and sometimes for predicting well performance. Since the mid-1990’s, there have been a number of behind-outcrop borehole imaging projects. In this paper we discuss two such projects involving turbidite and related strata, one in the Miocene Mount Messenger Formation of New Zealand, and the other in the Cretaceous Lewis Shale of Wyoming. In the Mount Messenger Formation, two boreholes were drilled 450 ft (150 m) apart and 360 ft (100 m) back from a well-exposed coastal cliff face. One borehole cored 140 ft (47 m) of strata, and the other borehole cored 315 ft (105 m). Both boreholes were logged using Schlumberger’s FMI™ and Platform Express™ log suites. By comparing the borehole images to the cores and outcrops, we were able to develop borehole-image criteria for a variety of sedimentary structures and stratification styles, and from these, to identify depositional facies. These recognition criteria were applied to the Ram-Powell “L” Sand reservoir in the offshore Gulf of Mexico and were found to be useful for identifying the character of the producing interval. In the Lewis Shale, a development well drilled 8 mi (13 km) west of Lewis Shale outcrops was logged through the same stratigraphic interval using Baker Atlas’ STAR™ acoustic and electrical borehole images, as well as conventional log suites using the ECLIPS™ acquisition and processing system. Again, a variety of sedimentologic features and bedding styles observed in the outcrop were identified on the image logs. In particular, laterally continuous and discontinuous sandstones could be differentiated from borehole image attributes, which allowed prediction of attributes away from the wellbore. Behind-outcrop borehole images from these two turbidite outcrops demonstrate their value for interpreting individual sedimentologic features, facies, and attributes away from the wellbore. These interpretations could not be made using conventional gamma-ray logs. Such observations and interpretations provided by borehole images are critical for improved volumetric calculations, well placement, and prediction of well performance.