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The Point Lookout Formation, which is well exposed along the northwest margin of the San Juan basin in northwestern New Mexico, includes nearshore sediments deposited during a regression of the Late Cretaceous epicontinental sea in earliest Montanan time. The unit is composed of sandstone and siltstone with sand percentage increasing up-section. Principal outcrop lithofacies include a lower interbedded, highly bioturbated, very fine sandstone and shale that represents the transition from innershelf to shoreface environments. The middle part of the unit gradationally overlies the lowermost lithofacies and represents a complex depositional history in the nearshore zone. These progradational units are thickly bedded and coarsen upward. This simple sequence is interrupted by he occurrence of hummocky stratified storm deposits and by broad surfaces of nondepositional scouring in the lower shoreface. A medium-grained upward-fining sandstone lithofacies that caps the entire formation has an erosional base and large-scale lateral accretionary bedsets.
Measured sections from the outcrop of the Point Lookout closely correspond with electric-log patterns from subsurface data east of the outcrop belt. The mapped distribution of SP-pattern facies (representing sandstone textural characteristics) depicts the primary depositional elements of the progradations. Correlation of genetically related sand packages permits the evaluation of changing sedimentation patterns through time. Seven regressive events (time-stratigraphic units) are recognized based on subsurface identification of the transgressive boundaries that rise stratigraphically away from the basin and obliquely traverse lithofacies boundaries. Each unit is composed of three depositional phases (progradation, transgression, and aggradation) that occur in regular succession.
Discrete distributary and interdistributary areas were maintained in the initial depositional phases throughout the history of the Point Lookout. In the broad areas between depositional axes the shoreline prograded by the seaward accretion of beach ridges until sediment sources became insufficient to maintain the shoreface advance. Transgressive reworking of the seaward part of the unit followed and dominated the arrangement of net-sandstone thicks by redistributing the sands into a strike-alignment. Each time-stratigraphic sedimentary unit is therefore the product of a progradational-transgressive depositional couplet. Whereas periodic transgressions were mainly erosive, they did cause the formation of coalesced shallow-shelf bars analogous to estuarine-shoal retreat massifs found on the modern continental margin of the Mid-Atlantic Bight.
During periods of shoreline stability following transgression a
channeled estuarine system developed landward of the retrograded shoreline trend. During this tide-dominated aggradational phase, channels migrated over the back-barrier area and produced the relatively coarser sand facies capping the Point Lookout in the study area. After the estuarine system was infilled, coastal plain facies were established in the former back-barrier zone and progradation was renewed. With the repetition of this depositional pattern through time, the coastal plain advanced in a step-wise fashion.
As a consequence of the progradational-transgressive cyclicity, a significant degree of stratigraphic rise was attained during the Point Lookout regression. Each time-stratigraphic coastal sand body acts as a discrete reservoir that interfingers landward with impermeable sediments of the coastal plain facies. Given the necessary present-day structural configuration, major stratigraphic rises corresponding to aggradational phases can act as updip migration boundaries to gas derived from the center of a basin.
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