Comparison of sedimentary structures in certain sedimentary rocks with those that form in the modern environment can provide a powerful interpretive tool. An example can be drawn from a well-exposed middle Miocene marine-nonmarine transition in the southeastern Caliente Range, California. Within this transition, shallow-marine sandstone (part of Branch Canyon Sandstone) intertongues on the northwest with marine siltstone (Saltos Shale Member of the Monterey Formation), and intertongues on the southeast with nonmarine redbeds (part of the Caliente Formation). Flow structure in extrusive basalts in the upper part of the transition and paleocurrent features in overlying Miocene alluvial deposits indicate that the shoreline at the time of deposition trended north-northwest. >

The marine-nonmarine transition consists of a succession of individual progradational sequences that extend westward (seaward) into marine strata. The sequences exhibit a fairly consistent internal stratigraphic arrangement. A typical complete sequence has a basal unit of unbedded siltstone (Saltos Shale) lying on an erosion surface. A thin zone of conglomerate commonly occurs within the siltstone directly above the lower contact. The siltstone grades up into bedded or unbedded fine-grained sandstone (Branch Canyon Sandstone). This fine sediment is sharply overlain by a coarse, pebbly, crossbedded facies of the Branch Canyon, which grades upward into finer and predominantly planar-bedded sandstone. The planar-bedded sandstone grades up into muddy structureless sandstone that in turn g ades up into red or green mudstone (Caliente Formation), which caps the sequence.

The progradational nature of the sequences implies that the fine-grained sandstone near the base was deposited in the marine environment somewhat shoreward from the gradationally underlying siltstone, which locally contains marine invertebrates. Bedding, where present in the fine sandstone, is defined by concentrations of biotite; the bedding is either planar or shows medium- and small-scale cross-stratification. Bioturbational disruption abounds. Crossbedded lenses of well-sorted granular sand are interbedded with the fine sand in its upper part. Foresets in these lenses dip predominantly toward the southeast. Similar structures form in response to the passage of waves in modern high-energy environments. The foreset orientation thus suggests that during the time of deposition the wav s approached from the northwest.

The coarse, pebbly sandstone that sharply overlies the fine-grained sandstone is the thickest and best exposed unit of most sequences. Bedding is generally well developed, and bioturbational structures are rare. Pebbles tend either to occur as lag deposits scattered along extensive erosion surfaces within the sandstone or to be concentrated within conglomeratic beds. Crossbedding is abundant and dips predominantly west-southwest (offshore); a small secondary mode dips south-southeast. This secondary mode may reflect the influence of longshore currents resulting from the oblique approach of the waves with respect to the shoreline. A few large-scale crossbedding units dip east and southeast and suggest the presence of offshore bars trending oblique to the coast, parallel with the prevai ing wave crests. The dominantly offshore-dipping crossbeds are best explained as the result of rip currents. The erosional contact at the base of the coarse sandstone resembles the contact formed at the base of rip channels in the modern environment.

The planar-bedded sandstone in the upper part of the sequence is well sorted and contains planar concentrations of magnetite that resemble those formed on the upper foreshore of modern beaches. Many planar laminations are inversely graded like those that form at present in the upper swash zone. Where attitudes were measured, most of the planar beds dip gently seaward (west), which also supports a beach origin.

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The overlying structureless muddy sandstone may represent deposition in a vegetated back-beach environment. The redbeds of the Caliente Formation probably formed in coastal swamps, lagoons, or alluvial plains.

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