Geology Reference
In-Depth Information
Plate 127 Problems with Standard Microfacies Types: Textural Changes in Bedded Limestones
Fluctuations in water energy and sediment transport can result in the formation of sedimentary layers differing
in depositional texture, constituent composition as well as the size and packing of grains. These differences
might give rise to SMF Type assignments that do not necessarily reflect the factual sedimentary environments.
To recognize these environments you need a combination of field data, paleontological criteria, and microfacies.
Small-scaled, short-time changes in depositional texture and the association of grain types are common in plat-
form interior areas and in inner and mid-ramp environments influenced by storm-induced currents. These changes
in texture and fabric differ from changes caused by turbidity currents and documented by distinct vertical suc-
cession patterns and wide areal extensions of sedimentary structures.
1
Changes in depositional energy.
The conspicuous characteristic of the sample is the alternation of cortoid grainstone (G)
and peloid packstone (P) fabrics. The lower grainstone layer (G1) and upper layer top (G3) consist of skeletal grains
(mostly corals - C, some dasyclad algae, DA, and a few mollusk shells, MS). Larger black grains are porostromate
oncoids (O). All skeletal grains are coated by micrite envelopes. Some grains are encrusted by foraminifera. Intergranular
pores are occluded by thin rims of early marine cements surrounding the grains, and late diagenetic cements (white)
occluding the remaining pore space. The intercalated packstone layer (2) consists of small peloids, most of which are
reworked and transported micrite clasts.
The coated grainstone shows criteria of SMF 11, the packstone of SMF 16-N
ON
-L
AMINATED
. The assignment to SMF 11
is supported by field data and paleontological criteria. The sample comes from bedded carbonates corresponding to
calcareous sands formed at the margin of a platform in a backreef position. An assignment of the packstone, however, to
SMF 16 would suggest tidal flat sedimentation and not reflect the genuine depositional environment. The packstone layer
originated from a very short high-energy event causing a redeposition of peloids from nearby protected areas. Note the
transitional character of the lower boundary of the packstone layer and the infilling of peloids in the underlying grain-
stone. White arrows point to geopetals (deposition of grains on top of shells). Strong shearing of the limestones is indi-
cated by different orientation and filling of microfractures. Late Triassic (Dachstein limestone, Norian): Loser, Styria,
Austria.
2
Storm sedimentation.
Transition of ooid grainstone (OG, 1) to bivalve shell- ooid rudstone (BOR, 2) at the top. Ooids are
conspicuously small and more or less the same size, indicating a pre-depositional sorting of the grains. The microstructure
of the shells shows reworking of ostreid bivalves. The blurred boundary between the two textures suggests that deposition
of both layers occurred during a single event. Ooids have settled down, coming to rest in the cup-shaped valves (note the
geopetals indicated by arrows), and filling shelter pores beneath the shells. The pores were occluded by calcite cement.
The upper part of the sample shows similarities with SMF 12-B
S
(bivalve shell hash). The dominance of ooids would
suggest an assignment to SMF 15 pointing to deposition in ooid shoals. However, the intimate relations between ooids
and shells as well as the chaotic orientation of the shell fragments characterize the sample as a storm deposit which can not
be categorized by a specific SMF Type. The sample comes from a storm-dominated inner ramp. Late Jurassic (Oxford-
ian): Osterwald, Lower Saxony, Germany.
3
Changes in water energy causing differences in grain packing
. The succession exhibits a lower packstone/bindstone
fabric (P/B, 1), separated by a distinct boundary (arrows) from an upper grainstone fabric (G, 2), which again grades into
a packstone/bindstone fabric (P/B, 3). The densely packed peloids, coated grains, oncoids and algae of the lower part are
bound together by microbial crusts. The grainstone of the upper part contains reworked fragments of porostromate algae
(PA) and gastropod shells (GA). Note oblique microfractures (MF) occluded with spar and gray silt.
The sharp boundary between the two textures suggests a more rapid stabilization and cementation of the bindstone
than the grainstone. The textural changes reflect short-term changes in water energy conditions. The packstone/bindstone
texture corresponds approximately to SMF 16, indicating deposition within a shelf lagoon (Facies Zone FZ 7). Paleonto-
logical data support this interpretation. The sample comes from an isolated Bahamian-type platform (see Pl. 134). Late
Jurassic (Tithonian): Sulzfluh, Graubünden, Switzerland.
4
Break in shallow-marine sedimentation.
The lower part of the succession (1) consists of variously sized, angular and
rounded lithoclasts embedded within a fine organic network formed by microbes and synsedimentary cements. The upper
part (3) exhibits a similar fabric but the lithoclasts are larger and of different composition. Some clasts are reworked
aggregate grains, others are extraclasts. The gray central part (2) consists of micritic cement, and differs from the lower
and upper layers by the absence of lithoclasts. The erosion at the top of the cement layer (arrows) and micro-encrusters
(ME) characterize a hardground evidencing sporadic breaks in deposition.
Attribution of the sample to SMF Types might lead to SMF 4 (microbreccia deposited on slopes) or SMF 5 (forereef
slope). Neither designation fits the real paleoenvironment. As shown by field data, the sediment originated in a near-coast
environment where semi-indurated muds with algal and aggregate grains were reworked and stabilized by microbial
mats. Lithoclasts are storm-transported chips. Late Cretaceous: Central Apennines, Italy.
