Robert W. Wellner

The Effect of Sea Level, Climate, and Shelf Physiography on the Development of Incised-Valley Complexes: A Modern Example From the East China Sea

ABSTRACT Analysis of high-resolution seismic reflection and sonar data from the East China Sea (ECS) continental shelf reveals the presence of an extremely broad (> 330 km) and deep (maximum incision of 72 m) incised-valley complex whose development and subsequent fill is constrained to the end of the last glacial cycle ( 20,000 yr BP to present). Results from this study indicate that sea-level change, climate-controlled discharge and sediment supply, and shelf physiography were important factors in the development and subsequent fill of the ECS incised- valley complex. For example, wet climatic conditions during the initial Marine Oxygen Isotope Stage (MIS) 2 fall of sea level promoted fluvial erosion of the exposed shelf. However, the trend toward drier conditions during the MIS 2 fall of sea level slowed the overall development of the incised-valley complex to the point where only minor incision occurred during the maximum lowstand. Furthermore, the low-gradient morphology of the exposed outer shelf diminished the ability of fluvial systems to incise. Instead, fluvial systems migrated laterally, creating a shallow ( 300 km) incised-valley system on the outer shelf. During the late MIS 2 rise of sea level, accommodation was generated within the flooded reaches of the valley complex. However, arid climatic conditions that prevailed at this time resulted in minor sediment delivery to the transgressive shoreline via the incised-valley complex. Thus, the upper part of the drowned lowstand fluvial deposits were exposed at the sea floor and reworked into a valley-wide tidal-bar and tidal sheet complex. Wetter climatic conditions during middle to late MIS 1 resulted in abundant sediment delivery to the drowned parts of the valley complex (estuary) and buried lowstand fluvial deposits below the depth of tidal ravinement, where they remained undisturbed. Current sequence stratigraphic models can be applied to the late Quaternary stratigraphic secession of the ECS. However, these models are too simplistic because they rely largely on rates and directions of sea-level change to explain stratal architectures. Although the extent and rate of sea-level change is extremely important, this study shows that high-frequency climate change and shelf physiography also play important roles in the development of stratigraphic architectures. These factors are particularly important for the development of incised valleys, where they directly influence incised-valley morphology and facies distributions.

Vertebrate Burrows from Triassic and Jurassic Continental Deposits of North America and Antarctica: Their Paleoenvironmental and Paleoecological Significance

Comparisons of recently identified Triassic and Jurassic continental trace fossils in North America and Antarctica to modern mammal and reptilian burrows facilitate the identification and interpretation of the ancient burrows as vertebrate in origin, indicating advanced behaviors. Hollow, bowl-shaped depressions in the Petrified Forest Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, are interpreted as nest-holes constructed possibly by phytosaurs, aeotosaurs, turtles, or rauisuchians. Large-diameter, multiple-branching, and interconnected burrows in the Owl Rock Member of the Chinle Formation in southeastern Utah are tentatively interpreted as vertebrate burrows indicating communal behavior. Complex, large-diameter burrows in the Salt Wash Member of the Upper Jurassic Morrison Formation near the Henry Mountains in southern Utah are interpreted as burrow systems of fossorial mammals. Large-diameter, gently dipping, simple, subhorizontal burrows in the Salt Wash Member are interpreted as possible dwelling burrows of sphenodontids. Other vertebrate trace fossils, such as the large-diameter burrows from the Lower Triassic Fremouw Formation in the Queen Maud Mountains of Antarctica, are reinterpreted as vertebrate burrows and were likely constructed by small mammal-like reptiles. These burrows were thought to have been enigmatic in origin and different from very large-diameter burrows interpreted as therapsid dwelling burrows. Descriptions and interpretations of all these trace fossils are important because most vertebrate ichnology research to date has focused on trackways or locomotion experiments with modern reptiles and birds. These Triassic and Jurassic ichnofossils represent fossorial and nesting behavior of several different groups of vertebrates. The Fremouw Formation burrows indicate fossorial behavior in several sizes of vertebrates, including small and large therapsids. The burrows were likely used for shelter, giving birth, raising young, and hibernation. During the early Mesozoic, the Fremouw landscapes were thought to have had cold winters due to their high-paleolatitude position. The bowl-shaped depressions from the Petrified Forest Member of the Chinle Formation likely represent the earliest known structures excavated by reptilians for the sole purpose of reproduction. Basic nest-hole architecture in extant reptiles with early Mesozoic ancestry has changed minimally in nearly 220 million years. Large-diameter, multiple branching, and interconnected burrows in the Owl Rock Member of the Chinle Formation were likely constructed by fossorial vertebrates that had communal family groups. Simple, gently dipping, subhorizontal burrows in the Morrison Formation were used by crocodiles or sphenodontids as dwelling structures constructed in firm, subaerially exposed substrata close to open bodies of water. Complex, large-diameter burrows in the Salt Wash Member suggest subsocial behavior of fossorial mammals, where the burrow was used for raising young, storage and disposal of food and wastes, and coping with episodic water inundation. Research demonstrates that fossorial behavior of reptiles, therapsids, and mammals was established by the beginning of the Mesozoic and prior to the break-up of Pangea. The basic architecture of vertebrate nest construction has changed little in nearly 280 million years. Fossorial burrowing behavior likely evolved several times in different vertebrate groups during this time. These basic burrow architectures are also used by invertebrate groups. This overlap in burrow architectures between vertebrates and invertebrates suggests strongly that paleoenvironmental and paleoclimatic organism-substrate relationships dictate the architecture used by the organism. These burrow morphologies indicate particular physicochemical conditions in terrestrial and freshwater settings that are unique to the continental realm.

CHONDRICHTHYANS FROM THE LOWER FERRON SANDSTONE MEMBER OF THE MANCOS SHALE (UPPER CRETACEOUS: MIDDLE TURONIAN) OF EMERY AND CARBON COUNTIES, UTAH, USA

Abstract The Lower Ferron Sandstone Member of the Mancos Shale in southeastern Utah preserves a chondrichthyan assemblage of at least 13 taxa that include: Hybodus sp., Ptychodus cf. P. mammillaris Agassiz, 1843, Ptychodus whipplei Marcou, 1858, cf. Chiloscyllium sp., Scapanorhynchus raphiodon (Agassiz, 1843), Cretodus crassidens (Dixon, 1850), cf. Leptostyrax sp., cf. Cretalamna appendiculata (Agassiz, 1835), Squalicorax sp., Pseudohypolophus mcnultyi (Thurmond, 1971), Protoplatyrhina hopii Williamson, Kirkland and Lucas, 1993, Ischyrhiza schneideri (Slaughter and Steiner, 1968), and Ptychotrygon triangularis (Reuss, 1844). Although this assemblage is typical of other Turonian chondrichthyan faunas in North America, fossil teeth are preserved in two unique facies associations that consist of arenitic sandstones with mud interclasts and rounded chert, feldspar, and quartz pebbles. The coarser beds within these facies associations are previously interpreted to represent storm events and turbidity flows associated with a sea level lowstand. Chondrichthyan teeth occurring within these coarser beds are indicative of extensive transport and reworking and attest to the durable nature of chondrichthyan teeth for biostratigraphic and paleoecological interpretations. Similar studies of chondrichthyan teeth in shelf marine settings may also provide new insights for facies interpretations related to sequence stratigraphy and regional stratigraphic correlations.

Sequence Stratigraphy and Reservoir Architecture of the Burgan and Mauddud Formations (Lower Cretaceous), Kuwait

A new sequence-stratigraphic framework is proposed for the Burgan and Mauddud formations (Albian) of Kuwait. This framework is based on the integration of core, well-log, and biostratigraphic data, as well as seismic interpretation from giant oil fields of Kuwait. The Lower Cretaceous Burgan and Mauddud formations form two third-order composite sequences, the older of which constitutes the lowstand, transgressive, and highstand sequence sets of the Burgan Formation. This composite sequence is subdivided into 14 high-frequency, depositional sequences that are characterized by tidal-influenced, marginal-marine deposits in northeast Kuwait that grade into fluvial-dominated, continental deposits to the southwest. The younger composite sequence consists of the lowstand sequence set of the uppermost Burgan Formation and transgressive and highstand sequence sets of the overlying Mauddud Formation. This composite sequence is sand prone and mud prone in southern and southwestern Kuwait and is carbonate prone in northern and northeastern Kuwait. The lowstand sequence set deposits of the Burgan Formation are subdivided into five high-frequency depositional sequences, which are composed of tidal-influenced, marginal-marine deposits in northeastern Kuwait that change facies to fluvial-dominated deposits in southwestern Kuwait. The transgressive and highstand sequence sets of the Mauddud Formation are subdivided into eight high-frequency, depositional sequences. The Mauddud transgressive sequence set displays a lateral change in lithology from limestone in northern Kuwait to siliciclastic deposits in southern and southwestern Kuwait. The traditional lithostratigraphic Burgan –Mauddud contact is time transgressive. The Mauddud highstand sequence set is carbonate prone and thins south- and southwestward because of depositional thinning. Significant postdepositional erosion occurs at the contact with the overlying Cenomanian Wara Shale. The proposed sequence-stratigraphic framework and the incorporation of a depositional facies scheme tied to the sequence-stratigraphic architecture allow for an improved prediction of reservoir and seal distribution, as well as reservoir quality away from well control.

Simple is better when it comes to sequence stratigraphy: The Clearwater Formation of the Mannville Group reinterpreted using a genetic body approach

Analysis of high-resolution three-dimensional seismic data from the Cold Lake Production Project (CLPP) of central Alberta, Canada, has resulted in a new sequence stratigraphic interpretation and depositional model for the upper Albian Clearwater Formation of the Mannville Group. Specifically, we document the presence of one sequence boundary within the Clearwater Formation that (1) separates older, deltaic deposits from a younger fluvial-dominated, terraced incised valley fill succession and (2) ties to a lowstand shoreline approximately 100 km (62 mi) to the north of the CLPP. Although this interpretation is far simpler than previous stratigraphic interpretations of this area, the sedimentologic record within the Clearwater Formation remains very complex because of the vertical stacking of high-energy fluvial to fluvial–estuarine deposits that are scour based. The composite sequence boundary identified here is associated with an extended period of landscape degradation and the formation of a moderately large valley that is complexly defined by a series of terraced fluvial deposits. Because individual channels eroded vertically and migrated laterally during both the fall and ensuing rise of sea level, the resulting valley-shaped stratigraphic sand body is (1) substantially wider than the true topographic valley (i.e., landform that is constrained by subvertical to near-vertical walls, open to the air, and typically resulting from degradation of the landscape via vertical and lateral erosion by a fluvial channel or channels) within which the lowstand channels flowed, (2) formed by both fluvial and marine processes that can be allogenic and/or autogenic in nature, and (3) defined by a composite surface that formed during the descending limb of a base level cycle and was partially modified during the subsequent base level rise and is thus of minor chronologic significance. We attempt to define the time of maximum topographic valley development, but younger erosion has removed much of the record of this valley. However, we estimate that the Clearwater Formation topographic valley had a maximum incision depth of greater than 60 m (>197 ft) and a width of approximately 20 km (12 mi). These dimensions correlate very well to incised valleys observed in the Quaternary. Analysis of core and log results within our seismic stratigraphic framework indicates that a fluviodeltaic model best explains the lithofacies distributions and geometries within the CLPP. Furthermore, finer-scale seismic mapping was used to encapsulate packages of sediment—which we refer to as genetic sedimentary bodies—whose reservoir properties could then be defined using core results. A genetic body approach to defining stratal architectures has resulted in (1) a predictive model for reservoir types and distributions across the CLPP; (2) accurate paleoenvironmental interpretations; and (3) a simple, yet robust sequence stratigraphic model of this area that is aligned with recent results reported from the study of similar systems in the Quaternary, recent morphologic observations from small-scale, physical sand box experiments, and the most up-to-date models of coastal fluvial erosion, deposition, and stratigraphic surface formation.