A r TE factos y educación en Mediart – Ricardo Forero C Mayo 2018 Imagen final

Laminae couplets in the Marca Shale have previously been interpreted as varves on the basis of the similarity of the lithology to recent laminated diatomaceous sediments (McGuire, 1988; Fonseca, 1997; Kodama & Ward, 2001). Fonseca (1997) added support to this assumption by performing mass accumulation rates and yielding figures very similar to those of modern coastal varves. The recognition here of a four part flux cycle, similar to that preserved in the Gulf of California, supports the varve interpretation. It is thus feasible to speculate on the seasonal timing of the individual flux events. By analogy with the ecology of modern forms, the mixed floral laminae of

Hemiaulus spp. and Stephanopyxis spp. chains, along with blooms of solitary Stellarima spp. are interpreted as late summer/autumn flux. Examination of early diagenetic nodules reveals that many of the valves were deposited intact, although blooms were subject to grazing. Therefore, valves must have been deposited en masse, consistent with a “fall dump” type event (Kemp et al., 2000). The near-monospecific laminae of Chaetoceros-type spores, interpreted as the self-sedimented aggregations from upwelling-fuelled blooms,are overwhelmingly indicative of spring bloom

Chapter 4 Sedimentology of the Marca Shale

terminal autumn/winter blooms have been recorded (Pike & Kemp, 1996b; Dean et al., 2001). It is unlikely that a spring bloom or upwelling signal would be completely absent from the sedimentary record and the Chaetoceros-type spore laminae are therefore interpreted as a spring upwelling signal. The modern homeophorm of A. morenoensis, Azpeitia has a peak flux in the summer (Romero et al., 2002). Deposition of the concentrations of A. morenoensis during the summer would be consistent with their position below the mixed floral assemblage deposited during the late summer/autumn. The intact nature of the valves implies grazing was bypassed and hence

deposition was rapid, likely related to the large size and robustness of the valves.

The season of deposition of terrigenous laminae conveys important climatic information and it is therefore an important aspect to try and resolve. Basins, such as the Santa Barbara basin, which are influenced by a temperate maritime climate, receive the highest terrigenous flux during the winter rainy period. In contrast, the Gulf of California experiences a monsoonal climate, and as such the highest terrigenous flux occurs during the summer through intense convective thunderstorms. Although P.sulcata is currently found concentrated in the water column during winter (Hobson & McQuoid, 1997), this is likely related to strong physical mixing and does not help constrain the timing of deposition of terrigenous laminae. However, the interpretations of the season of

deposition of the three diatomaceous laminae types confine deposition of the terrigenous laminae to the summer.

This observation is unexpected as peak levels of precipitation at 43°N would conventionally be expected to occur during the winter in response to prevailing surface westerlies. Some of the few climate models for the Maastrichtian suggest that the Californian margin was characterised by wet winters and dry summers (e.g. Barron et al., 1993). One possible explanation is that deposition actually occurred at lower latitudes and the palaeolatitudinal reconstruction of Fonseca (1997) is incorrect. Palynological data suggests the Maastrichtian Californian margin adjacent to the San Joaquin basin had a vegetation cover similar to that of modern day Guatemala and Mexico (Gulbrandsen et al., 1963; Drugg, 1967), which are at present influenced by the ITCZ and characterised by a pronounced monsoonal wet season between May and October. The ~43°N reconstruction of Fonseca (1997) was based on the Late Cretaceous dipoles of Diehl (1991) and Gunderson and Sheriff (1991) from studies in Montana and did not take into consideration the possibility of any strike slip movement. Therefore, if any considerable translation by strike slip faulting occurred during or following emplacement of the terrane, the ~43°N reconstruction would be invalid. Evidence for such strike slip translation is not forthcoming, however

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communication between the Pacific and Tethys Oceans may have permitted dense, saline, tropical waters from the equatorial current to invade polewards during periods of maximum insolation (Bush & Philander, 1997), which would have been accompanied by intense low pressure systems,

Chapter 4 Sedimentology of the Marca Shale

can be corroborated with evidence of a northwards flowing current along the Californian margin (Anderson, 1958; Gordon, 1973; Johnson, 2002), a reduced meridional temperature gradient (Amiot et al., 2004) and extreme polar warmth (Spicer & Parrish, 1990; Herman & Spicer, 1997; Francis & Poole, 2002; Jenkyns et al., 2004; Poole et al., 2005). Recent analysis of the output of 15 coupled climate models reveals a consistent poleward shift of storm tracks with increased

atmospheric ρCO2, associated with poleward shifts in surface wind stress and precipitation (Yin,

2005). A poleward shift of northern hemispheric tropical precipitation patterns could also account for summer terrigenous deposition.

Maximal fluvial runoff during the summer could therefore indicate that the 43°N palaeolatitude estimate is incorrect. Alternatively, summer terrigenous deposition could have been in response to intense monsoonal storms, resulting from the globally elevated temperatures, shallower meridional temperature gradient and presence of a poleward flowing reverse coastal current along the Late Cretaceous Californian margin and the possible poleward shift of storm tracks. Although such storms are of short duration, they are often associated with very high levels of precipitation and runoff. Valves of A. morenoensis often occur within the top of terrigenous laminae, implying that they must have been deposited during the end of the wet season. The analogy of Azpeitia with

Azpeitiopsis is consistent with these observations. A. nodulifera is brought into the modern Gulf of California by invading tropical waters which generate the intense summer storms. If A.

morenoensis was also periodically brought into the basin by invading tropical waters then it would be expected to occur within terrigenous laminae.