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Despite the potential of sandstone-filled normal faults to significantly influence fluid transmissivity within reservoirs and the shallow crust, they have to date been largely overlooked. Fluidized sand, forcefully intruded along normal fault zones, markedly enhances the transmissivity of faults and, in general, the connectivity between otherwise unconnected reservoirs. Here, we provide a detailed outcrop description and interpretation of sandstone-filled normal faults from different stratigraphic units in central California. Such faults commonly show limited fault throw, cm to dm wide apertures, poorly-developed fault zones and full or partial sand infill. Based on these features and inferences regarding their origin, we propose a general classification that defines two main types of sandstone-filled normal faults. Type 1 form as a consequence of the hydraulic failure of the host strata above a poorly-consolidated sandstone following a significant, rapid increase of pore fluid over-pressure. Type 2 sandstone-filled normal faults form as a result of regional tectonic deformation. These structures may play a significant role in the connectivity of siliciclastic reservoirs, and may therefore be crucial not just for investigation of basin evolution but also in hydrocarbon exploration.

The Tumey Giant Injection Complex (TGIC) is a regionally developed sandstone intrusion complex emplaced into the deep-water Kreyenhagen Shale (Eocene) in the San Joaquin Basin, Central California. Detailed geological mapping, stratigraphic reconstruction and outcrop description, supported by structural analysis, allowed the architectural characterization of the TGIC. The complex is described as two main stratigraphically constrained intervals: (1) a lower interval (250 m thick) emplaced into clay-rich mudrock, consisting dominantly of sills with stepped and multilayered geometry; and (2) an upper interval (200 m thick) characterized by injection breccia and large wing-like intrusions (c. 600 m width × 100 m high) emplaced within predominantly biosiliceous mudrock strata. The intrusions in both intervals were derived from turbiditic channel fills intensely modified by sand fluidization. Sandstone intrusions and fractures affecting host strata are dominantly oriented sub-parallel to the basin axis striking between NW–SE and N–S, mainly dipping to NE and forming asymmetric saucer-shaped intrusions, suggesting structurally driven hydraulic fracturing and sand emplacement. The absence of a deep aquifer and potential sand sources underlying the complex suggests a lateral contribution of fluid flow. The TGIC occurs at a scale similar to injection complexes recognized in the subsurface and is a valuable reservoir analogue for hydrocarbon accumulations associated with sand injectites.

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The proposed geological itinerary connects a series of impressive outcrops distributed between the front and the inner portion of the Southern Apennine thrust belt. It offers the opportunity to visit some of the most interesting and bestexposed Southern Apennine geosites, inserted within landscapes typical of the different sectors of the thrust belt, in order to illustrate its geological evolution. The itinerary has been designed in such a way as to intersect different stacked tectonic units and a great variety of sedimentary successions whose age ranges from Triassic to Pleistocene. Each of the chosen geosite contains multidisciplinary geological information that may be of interest for researchers, but may be also appreciated by a general public interested in geology. In particular, the stratigraphy, the structural geology and the geomorphology are the most recurrently treated topics. Most of these outcrops extend for many hundreds or thousands of metres and include villages or mountain reliefs. Therefore, every single site can be considered as an areal geosite containing particular geological features. However, the largest geosite can also be observed in a panoramic view from suitable locations. Thus, each of these sites provides different evidence and when combined together provides an opportunity to understand the complex geological history of the Southern Apennines. The motivation for establishing this itinerary mostly resides in the great scientific interest of the chosen outcrops; this will hopefully facilitate their conservation and the development of geotourism.