Reinterpreting the Paleosalinity and Water Depth of Peripheral Foreland Basin Flysch
Countering the idea that every peripheral foreland basin (PFB) has an early deep-water stage, recent papers and my observations show (1) Taiwan PFB (6.5-0Ma) has never been deep and (2) many ‘external flysch’ formations (e.g. Annot, Brushy, Bude, Hecho, Jackfork, Marnoso, Ross, Skoorsteenberg, Toro) have recurrent event beds with evidence for waves (e.g. HCS; near-symmetric ripples). The following non-actual model explains this and why PFB flysch basins inherently fluctuate from marine to fresh. When a long (1,000s km) continent collides with another (e.g. Alpide, Gondwanide, Variscan orogens) or an island arc, collision occurs first at a salient. Thrust mountains (carrying ‘internal flysch” scraped off ocean crust) mount the incoming passive-margin shelf, initiating a PFB on it. The PFB and mountains lengthen by diachronous collision and advance until suturing occurs (progressively later strikewise). The initial PFB is a shelf-depth strait (e.g. Taiwan), merging axially with the passive shelf. The strait can deepen only if sedimentation < subsidence (cf. 2.5-0Ma Timor Trough, >2km deep due to scant supply from arid Australia). Eventual high supply from the growing mountains builds an alluvial neck (tombolo at first), splitting the strait into back-to-back gulfs (blind shelves). The forebulge runs down each gulf, crosses the adjacent shelf and slope obliquely (thus annexing a ‘shelf triangle’ to the PFB), and runs along the remnant ocean as a trench outer rise. Gradual along-shelf migration of the bulge and gulf forms an unconformity onlapped by gulf/triangle flysch (see below). Collision at a 2nd salient pinches off a remnantocean sector, forming an ‘ocean lake’, dammed by a 2nd tombolo (unless one collider is a gapped island arc). If river inflow > evaporation the lake freshens and overflows, carving a trans-dam spillway. Eustatic rises over the spillpoint (SP; in spillway) raise the lake and, if high enough to admit an ocean wedge, raise the salinity (to marine if wedge height and width suffice). Eustatic falls below SP leave the lake perched (at SP), freshening. This curtailment of falls (‘sill-damped eustasy’) means only the innermost gulf is forcibly exposed, so even at lowest lowstand (SP) the gulf remains long (>100km); it cannot shorten by gulf-head-delta progradation as the lake is then freshest and distributaries incised deepest, both maximizing hyperpycnal delta bypass. Megaflood hyperpycnal events feed ‘shallow flysch’ to the gulf and triangle, mainly Bouma, Lowe and storm-wave-modified beds. ‘False bathyal’ foram assemblages reflect internal-flysch forams reworked by rivers in flood and deposited in hypo/ meso/hyperpycnal mud; plus ‘slope mimicking’ by the gulf during highest highs (i.e. marine; mud floor; dysoxia by thermohaline stratification). Many so-called hybrid beds, slurries and slumps are seismites, in situ or nearly. The forebulge may supply carbonate-rich tsunamites into the gulf. Shallow-flysch hitherto enigmatic alternation of thinner- and thicker-bedded (coarser) packets 1-30m thick is eustatic T-R cyclicity; the inter-packet leap in average bed thickness (e.g. cm vs dm) shows that rises and falls are brief (inter-event) yet large enough to greatly alter proximality, i.e. they are very fast, e.g. 20m rise or fall in 0.5ka (like Quaternary glacioeustatic solar cycles) moves the delta mouth 20km on a 1:1000 gulf gradient. Lack of inter-packet evidence for winnowing (erosion) or condensation (drowning) means the gulf axial gradient is near-linear (‘storm-graded’ equilibrium profile, intrinsic to tideless shelves free of forced exposure), so any rise or fall alters depth (wave power) and proximality equally (%), thus the grain-size arrays (background and event) simply shift in- or out. At stillstand (hi-, low), each megastorm shaves off a layer (cm-dm; swept over shelf edge), maintaining the equilibrium profile, preventing emergence. Shallow flysch interfingers upflow with highstand delta-slope muddy clinothems; and orogenward with olistostromes. Shallow-flysch sand bodies (point-fed hyperpycnite ovoids cut by hyperpycnitic slope-canyon tributaries) are bad ‘outcrop analogs' for fully marine, truly deep-sea, leveed-channel and fan oil reservoirs.
AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015