ABSTRACT In 1935, Krynine postulated that first-cycle arkose in the humid tropical setting of southern Mexico can be rapidly eroded with minimal chemical weathering and redeposited as second-cycle arkose. Modern quantitative data confirm this hypothesis and highlight exceptions where first-cycle arkosic sediments have been diagenetically altered by intense weathering to yield second-cycle quartz arenites. In this study, extensive sampling of upland source rocks and their derived sediments provided a robust data set with which to quantitatively evaluate the composition and provenance of Holocene sediments. Three upland source terrains were identified: Paleozoic crystalline basement of the Chiapas Massif; Mesozoic to Cenozoic siliciclastic and carbonate rocks of the Chiapas fold belt; and Cenozoic sedimentary rocks in the foothills of the fold belt. Holocene sediments from these source terrains are grouped into seven facies (A–G) based on their provenance and geographic location. Facies A consists of feldspathic sediments from the Mezcalapa-Grijalva River that are sourced from the Chiapas Massif. Facies B consists of lithic-rich sediments from the same area that are derived from the Chiapas fold belt. Facies A and B consist predominantly of first-cycle sand capable of yielding arkosic deposits. Facies C represents a mixture of Facies A and B sands deposited along the course of the Mezcalapa-Grijalva River. Facies D (from Rio Sierra) and Facies E (from Rio Pedregal) represent second-cycle feldspathic sands of the coastal-plain delta and were derived from Cenozoic sedimentary rocks of the foothills. Mild chemical weathering due to rapid mechanical erosion enabled the creation of these arkosic deposits. They are less feldspathic than their parents and have limited occurrence due to mixing with less feldspathic first-cycle sands downstream from their sources. Facies F (from Rio Zanapa) and Facies G (from Lagunas Rosario and Enmedio) represent second-cycle quartzose sands of the low-lying savanna that were also derived from Cenozoic sedimentary rocks in the foothills of the fold belt. Intense, long-term (>10,000 yr) chemical weathering of these sands has precluded the formation of arkoses, instead yielding quartz arenites. They are more weathered than the delta sands (Facies D, E) with a greater loss of feldspar and carbonate detritus. They are enriched in silica and depleted in alumina, CaO, Na 2 O, and K 2 O relative to Facies A arkoses due to loss of feldspars and mafic minerals. Second-cycle sediments eroded from Tertiary sedimentary rocks in the foothills (Facies D–G) contain detrital serpentine and chromite with high abundances of Cr and Ni, suggesting an ultramafic component in their provenance. Cr and Ni are effective tracers for second-cycle components in sands of mixed provenance.

Abstract The stratigraphic and lateral distributions of authigenic minerals in feldspar-rich Paleogene sandstones of the Santa Ynez Mountains, California, are important in determining their reservoir quality. The sandstones were deposited in an east-west elongate basin during two regressive episodes. Deep-water turbidites were overlain by shallow-water traction deposits and eventually by continental fluvial deposits as the basin was progressively filled from the east. Modal analyses document a common provenance for all the Paleogene sandstones consisting primarily of acidic to intermediate plutonic rocks, with minor volcanic, metamorphic, and sedimentary components. The average detrital mode of 27 sandstones is Q 37 F 54 L 9 , and the average partial mode including only the monocrystalline mineral grains is Qm 39 P 40 K 21 . Textural relationships and the stratigraphic distribution of diagenetic minerals delineate the paragenetic sequence: (1 ) syndepositional to very early pyrite; (2) early concretionary calcite cement; (3) incipient dissolution of detrital heavy minerals and feldspars; (4) clay pore linings and pore fillings; (5) formation of sphene and anatase; (6) incipient albitization of detrital plagioclase; (7) quartz, plagioclase, and K-feldspar overgrowths; (8) dissolution of feldspar creating secondary porosity; (9) local precipitation of pore-filling kaolinite; (10) laumontite cementation and replacement of plagioclase; (11) barite cementation and replacement of detrital grains; and (12) late-stage calcite replacement of detrital grains and earlier cements. Organic metamorphism, as expressed by vitrinite reflectance (R O ), provides a means to correlate mineral diagenesis in the sandstones with the thermal history of the Santa Ynez basin. In the eastern end of the basin (Wheeler Gorge) incipient albitization is first recognized at 0.5% R O corresponding to a paleotemperature of 110°C (4572 m burial depth), with complete albitization first occurring at a reflectance of 0.90% R O corresponding to a paleotemperature of 165°C (5425 m burial depth). The first occurrence of laumontite is in the turbidite beds of the basal Matilija Formation (5669 m burial depth) at approximately 1.0% R 0 reflectance (173°C). Further to the west, at Point Conception (Gerber No. 1 well), the first occurrence of laumontite is at an estimated burial depth of only 2515 m, corresponding to approximately 0.5% R 0 and a paleotemperature of 110°C. In this well, incipient albitization begins at 0.35% R O (77°C), with complete albitization occurring at roughly the same burial depth (2515 m) and reflectance (0.5% R O ) as the first occurrence of laumontite. The top of the laumontite zone occurs at greater burial depths and paleotemperatures in the eastern portion of the Santa Ynez basin than in the west. Laumontite distribution appears to be controlled by porefluid chemistry and post-compaction permeability variations, which are responsible for creating differences in fluid pressure between petrologically similar sandstones. “Dynamic” overpressuring may have occurred in the turbidite facies of the Juncal and lower Matilija Formations, whereby pore fluids enriched in Na + from the dewatering of smectite-rich shales permeated into the turbidite sandstones at a faster rate than they were expelled. Under these conditions, a continuous supply of Na + would have been delivered to the sandstones to allow albitization of calcium-bearing plagioclase, which in turn supplied Ca +2 necessary for the formation of laumontite. The authigenic minerals in the lower Paleogene sandstones of the Santa Ynez Mountains render them ineffective as reservoirs. Better reservoir prospects occur in the upper Paleogene and Neogene sandstones, particularly in the western part of the basin where they have not been subjected to deep burial, and secondary porosity is well developed.