Abstract The Cretaceous Western Interior Seaway, North America, contains numerous coastal and shelf sandstone bodies that are interesting from a commercial and academic viewpoint. Exploration and production of hydrocarbons has provided an enormous amount of subsurface data that supplement superb outcrops, and many excellent papers on detailed sedimentology and sequence stratigraphy have been produced. However, consensus regarding interpretation of some of the sandstones is still out of sight. This paper proposes consideration of yet another possible interpretation, namely large sandy spit systems. The Western Interior Seaway was characterized by recurrent sea-level changes, long-distance shifts in coastline position, incised valleys, large influxes of sand from rivers building deltas, strong waves that caused longshore sand drift toward the south, and differential subsidence and uplift that occasionally fragmented the basin into subbasins with shoals and islands. This physiography probably favored formation of large sandy, southward-prograding spit systems; however, these have not yet been recognized, possibly due to poorly constrained paleography, partial preservation, and lack of suitable facies models. This paper thus presents two well-constrained spit-system models based on well-preserved, uplifted Quaternary spit systems with exposures of the internal facies architecture and present-day morphology showing growth patterns. The Lyngså spit system (10 km long, 0.5-2.5 km wide, and up to 15 m thick) was formed in less than 500 years in a moderate-wave-energy, nontidal to microtidal environment, and it prograded into 10-15 m of water depth. The Skagen spit system provides a unique possibility for comparing preserved structures with processes, inasmuch as its old proximal part is uplifted 13 m and exposed in seacliffs, while its distal end is still prograding. The spit system (22 km long, 3-7 km wide, and up to 32 m thick) has been active for 5,500 years and is situated in a high-energy, wave-dominated, nontidal environment and has prograded into 20-30 m of water depth. In both systems the wave energy was transformed into strong longshore currents when the waves hit the mainland and spit coasts. Thus current-generated structures dominate both systems, whereas wave-formed structures are less common. Both systems overlie offshore mud and silt. The Lyngså system consists of coarse-grained, steep avalanche-type platform foresets, up to 10 m high, overlain by bar-trough deposits, beach sand, and gravel. The Skagen system consists of gently dipping, sandy spit-platform clinoforms composed of bioturbated storm sand beds, dune cross-beds and ripples, beach sand and pebbles, and peat beds. Both systems are comparable in size to proven hydrocarbon reservoirs, and their dimensions and structures resemble some of the sandstone ridges in the Cretaceous Seaway.

From Pliocene to middle Pleistocene time, a large lake occupied most of the San Luis Valley above 2300 m elevation (7550 ft) in southern Colorado. This ancient lake accumulated sediments of the Alamosa Formation (Siebenthal, 1910), for which the lake is herein named. The existence of this lake was first postulated in 1822 and proven in 1910 from well logs. At its maximum extent of nearly 4000 km 2 , it was one of the largest high-altitude lakes in North America, similar to but larger than Lake Texcoco in the Valley of Mexico. Lake Alamosa persisted for ~3 m.y., expanding and contracting and filling the valley with sediment until ca. 430 ka, when it overtopped a low sill and cut a deep gorge through Oligocene volcanic rocks in the San Luis Hills and drained to the south. As the lake drained, nearly 100 km 3 (81 × 10 6 acre-ft or more) of water coursed southward and flowed into the Rio Grande, entering at what is now the mouth of the Red River. The key to this new interpretation is the discovery of ancient shoreline deposits, including spits, barrier bars, and lagoon deposits nestled among bays and in backwater positions on the northern margin of the San Luis Hills, southeast of Alamosa, Colorado. Alluvial and lacustrine sediment nearly filled the basin prior to the lake's overflow, which occurred ca. 430 ka as estimated from 3 He surface-exposure ages of 431 ± 6 ka and 439 ± 6 ka on a shoreline basalt boulder, and from strongly developed relict calcic soils on barrier bars and spits at 2330–2340 m (7645–7676 ft), which is the lake's highest shoreline elevation. Overtopping of the lake's hydrologic sill was probably driven by high lake levels at the close of marine oxygen-isotope stage (OIS) 12 (452–427 ka), one of the most extensive middle Pleistocene glacial episodes on the North American continent. Hydrologic modeling of stream inflow during full-glacial-maximum conditions suggests that Lake Alamosa could fill at modern precipitation amounts if the mean annual temperature were just 5 °C (10 °F) cooler, or could fill at modern temperatures with 1.5 times current mean annual precipitation. Thus, during pluvial epochs the lake would rise to successively higher levels owing to sedimentation; finally during OIS 12, the lake overflowed and spilled to the south. The integration of the upper (Colorado) and lower (New Mexico) reaches of the Rio Grande expanded the river's drainage basin by nearly 18,000 km 2 and added recharge areas in the high-altitude, glaciated San Juan Mountains, southern Sawatch Range, and northern Sangre de Cristo Mountains. This large increase in mountainous drainage influenced the river's dynamics downstream in New Mexico through down-cutting and lowering of water tables in the southern part of the San Luis Valley.

Abstract The Troll Field in the Norwegian North Sea is one of the largest offshore gas fields in the world. Its western part contains a thin but exploitable oil leg (11–26 m) below the thick gas column, with the majority of the oil located in the late Jurassic Sognefjord Formation. The reservoir geology of the Troll Field was discussed in a few papers prior to production start-up in 1995, but a comprehensive account of the geological model of the Late Callovian – Late Oxfordian Sognefjord Formation has not as yet been published. The present paper reviews the depositional setting and architecture of this reservoir unit based on integration of results from disciplines such as sedimentology, seismic data analysis and stratigraphy. The Troll West reservoir formed on the edge of the Horda Platform during the Late Jurassic rift event; it consists of numerous stacked and generally offlapping sandstone units with intervening finer-grained deposits. The reservoir succession contains three composite sequences, of which the lower two belong to the Sognefjord Formation and the upper one is part of the Draupne Formation. Within this framework, five basic sequences and fifteen reservoir zones occur at the systems tract scale. The lower to middle parts of the studied succession reflect southwestwards growth and decay of a coastal spit system flanked to the east by a tidal backbasin. Brackish water facies have been identified in the eastern parts of the field through the use of detailed palynology. The extensive well database (including 15 cored production wells) and the high quality of the seismic data in the Troll area provide a unique opportunity to gain insight into the evolution of such a major spit-backbasin to tide-dominated deltaic system. A typical spit/strandplain progradation episode results in a clinoform succession comprising bioturbated sands of offshore transition origin overlain by lower shoreface sandstones which pass upward into clean, generally coarser-grained sands of upper shoreface and foreshore origin. The bases of such regressive sandbodies are often sharp, due to rapid facies translations during forced regression, but they may also be characterized by up to 40 m thick coarsening-upwards successions containing alternating siltstones and sandy event-beds generated by storm winnowing of the spit platform. Eastwards, these shallow-marine sandstone units finger into heterogeneous coastal plain deposits. On seismic data, this lateral transition is portrayed by the change from clinoform units in the west to undulatory seismic patterns in the east, as seen on maps based on classification of trace shape. A change in coastal morphology from wave- to tide-dominated took place in the Troll West area in the Late Oxfordian, perhaps related to an increase in tidal range. In the tidal facies tract, heterolithic inshore deposits formed in tidal channels and flats are replaced, in a seaward direction, by more sand-dominated high-energy tidal deposits such as mouth-bars and sand-ridge complexes. Further seawards, muddy shelf or prodelta deposits similar to those found in the wave-dominated facies tract are seen. In the Kimmeridgian, relative sea-level rise and eastward tilting of the Horda Platform caused reworking of the upper part of the Sognefjord Formation and localized hanging-wall shoreline sands formed as the uppermost reservoir intervals on Troll West.

Abstract: The Godavari delta, on the east coast of India, is prograding into a microtidal and low to moderate wave environment, fed by a highly variable water discharge in a monsoon-driven hydrographic regime. Extensive strandplains that prograded 30 to 35 km across the continental shelf during the Middle to Late Holocene characterize the overall arcuate seaward bulge of the delta. At present, the delta has two lobes: the Gautami in the northeastern part and the Vasishta in the southwestern part, which have been active during the past one thousand years. Although the bigger size of the Gautami lobe with extensive mangrove swamps apparently indicates a larger riverine influence, both lobes, in fact, exhibit strong wave-influenced morphologies. As revealed by a series of historical maps, pronounced growth of spits characterizes the northern Gautami lobe. The 21-km-long Kakinada spit, which is prominent among all such sand bodies, had even deflected the Gautami distributary course in the initial stages of its growth under the influence of a strong northeastward longshore drift. Analysis of multi-date satellite images shows that spits are growing at the updrift sides of the mouths of the two terminal branches of Gautami, whereas erosion is dominant on their downdrift sides. On the downdrift side of the Vasishta mouth, development of mouth bars is followed by their emergence as barrier islands that migrate landward, and subsequent infilling of the backbarrier bays by riverine discharge, typical for wave-influenced asymmetric deltas. The nature of coastal landforms and sedimentation processes indicate prevalence of a northeastward drift at the Gautami lobe and a southwestward drift at the Vasishta lobe.